docs(apple): Add standalone app start tracing docs

docs/platforms/apple/common/configuration/options.mdx

@@ -106,8 +106,7 @@ You can also override this option on a per-call basis by passing the `attachAllT
 
 When enabled, the SDK reports SIGTERM signals to Sentry.
 
-It's crucial for developers to understand that the OS sends a SIGTERM to their app as a prelude to a graceful shutdown, before resorting to a SIGKILL. This SIGKILL, which your app can't catch or ignore, is a direct order to terminate your app's process immediately. Developers should be aware that their app can receive a SIGTERM in various scenarios, such as  CPU or disk overuse, watchdog terminations, or when the OS updates your app.
-
+It's crucial for developers to understand that the OS sends a SIGTERM to their app as a prelude to a graceful shutdown, before resorting to a SIGKILL. This SIGKILL, which your app can't catch or ignore, is a direct order to terminate your app's process immediately. Developers should be aware that their app can receive a SIGTERM in various scenarios, such as CPU or disk overuse, watchdog terminations, or when the OS updates your app.
 
 </SdkOption>
 
@@ -205,7 +204,6 @@ This option can be overridden using <PlatformIdentifier name="inAppInclude" />.
 
 A block that configures the initial scope when starting the SDK.
 
-
 ```swift
 import Sentry
 
@@ -301,6 +299,7 @@ Set this boolean to `false` to disable sending of client reports. Client reports
 When enabled, the SDK adds breadcrumbs for each network request. This feature uses swizzling, so disabling <PlatformLink to="/configuration/swizzling/">`enableSwizzling`</PlatformLink> also disables this feature.
 
 If you want to enable or disable network tracking for performance monitoring, use <PlatformLink to="/configuration/options/#enableNetworkTracking">`enableNetworkTracking`</PlatformLink> instead.
+
 </SdkOption>
 
 <SdkOption name="enableCaptureFailedRequests" type="bool" defaultValue="true">
@@ -640,7 +639,6 @@ If <PlatformIdentifier name="tracePropagationTargets" /> is not provided, trace
 
 </SdkOption>
 
-
 <SdkOption name="strictTraceContinuation" type="Bool" defaultValue="false">
 
 If set to `true`, the SDK will only continue a trace if the organization ID of the incoming trace found in the `baggage` header matches the organization ID of the current Sentry client.
@@ -673,7 +671,6 @@ Use <PlatformIdentifier name="tracePropagationTargets" /> to control which reque
 
 <SdkOption name="enablePersistingTracesWhenCrashing" type="bool" defaultValue="false" availableSince="8.41.0">
 
-
 <Alert>
 
 This feature is experimental and may have bugs. It's available from [version 8.41.0](https://github.com/getsentry/sentry-cocoa/blob/main/CHANGELOG.md#8410).
@@ -852,8 +849,6 @@ When enabled, request and response bodies are captured for URLs matching `networ
 
 Requires `options.experimental.enableReplayNetworkDetailsCapturing = true`.
 
-
-
 </SdkOption>
 
 <SdkOption name="sessionReplay.networkRequestHeaders" type="array" defaultValue='["Content-Type", "Content-Length", "Accept"]' availableSince="9.12.0">
@@ -1108,6 +1103,7 @@ Learn more in the <PlatformLink to="/features/experimental-features/">Experiment
 <SdkOption name="cacheDirectoryPath" type="string">
 
 The path where the SDK stores data before sending it to Sentry. By default, the SDK uses `NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, true)`, which resolves to:
+
 - `~/Library/Caches/` for non-sandboxed macOS apps
 - `~/Library/Containers/{BundleID}/Data/Library/Caches/` for sandboxed macOS apps
 - `~/Library/Caches/` (within the app's sandbox) for iOS apps
@@ -1186,6 +1182,15 @@ Learn more in the <PlatformLink to="/session-replay/configuration/#network-detai
 
 </SdkOption>
 
+<SdkOption name="experimental.enableStandaloneAppStartTracing" type="bool" defaultValue="false" availableSince="9.14.0">
+
+When enabled, the SDK sends app start data as a standalone `app.start` transaction instead of attaching it as spans to the first UIViewController transaction. This makes app starts easier to find, sample, and analyze independently.
+
+Learn more in the <PlatformLink to="/tracing/instrumentation/automatic-instrumentation/#standalone-app-start-tracing">Standalone App Start Tracing</PlatformLink> documentation.
+
+</SdkOption>
+
+
 <SdkOption name="experimental.enableMetrics" type="bool" defaultValue="true" availableSince="9.4.0" removedSince="9.12.0">
 
 **Removed in version 9.12.0** — Metrics has graduated to GA. Use the top-level <PlatformIdentifier name="enableMetrics" /> option instead. See [sentry-cocoa#7842](https://github.com/getsentry/sentry-cocoa/pull/7842).

docs/platforms/apple/common/features/experimental-features.mdx

@@ -26,6 +26,12 @@ Enable <PlatformLink to="/profiling/#enable-launch-profiling">App Launch Profili
 
 Enable <PlatformLink to="/profiling/#continuous-profiling">Continuous Profiling</PlatformLink> to get full coverage of your app's execution.
 
+## Performance Features
+
+### Standalone App Start Tracing
+
+Enable <PlatformLink to="/tracing/instrumentation/automatic-instrumentation/#standalone-app-start-tracing">`enableStandaloneAppStartTracing`</PlatformLink> to send app start data as a dedicated transaction instead of attaching it to the first UIViewController transaction. You can use a custom <PlatformLink to="/configuration/options/#tracesSampler">`tracesSampler`</PlatformLink> to set a dedicated sample rate for app start transactions by checking for the `app.start` operation.
+
 ## Crash & Error Handling
 
 ### Persisting Traces When Crashing

docs/platforms/apple/common/tracing/instrumentation/automatic-instrumentation.mdx

@@ -134,6 +134,7 @@ SentrySDK.start { options in
     options.enableAutoPerformanceTracing = false
 }
 ```
+
 ```objc {tabTitle:Objective-C}
 @import Sentry;
 
@@ -166,6 +167,7 @@ The Cocoa SDK creates the following app start spans:
 ![App Start Transaction](./img/app-start-transaction.png)
 
 Cold and warm start are Mobile Vitals, which you can learn about in the [full documentation](/product/dashboards/sentry-dashboards/mobile/mobile-vitals).
+
 ### Prewarmed App Start Tracing
 
 Starting with iOS 15, the operating system might [prewarm](https://developer.apple.com/documentation/uikit/app_and_environment/responding_to_the_launch_of_your_app/about_the_app_launch_sequence#3894431) your app by creating the process before the user opens it. Starting with SDK version [9.0.0](https://github.com/getsentry/sentry-cocoa/releases/tag/9.0.0), the `enablePreWarmedAppStartTracing` is enabled by default.
@@ -201,6 +203,70 @@ SentrySDK.start { options in
 }];
 ```
 
+### Standalone App Start Tracing
+
+<Alert>
+
+This feature is experimental and available since [version 9.14.0](https://github.com/getsentry/sentry-cocoa/blob/main/CHANGELOG.md#9140).
+
+</Alert>
+
+By default, the SDK attaches app start data as spans to the first UIViewController transaction. With standalone app start tracing enabled, the SDK sends a dedicated `app.start` transaction instead. This gives app starts their own transaction, making them easier to find, sample, and analyze independently.
+
+![Standalone App Start Transaction](./img/standalone-app-start.png)
+
+To enable standalone app start tracing:
+
+```swift {tabTitle:Swift}
+import Sentry
+
+SentrySDK.start { options in
+    options.dsn = "___PUBLIC_DSN___"
+    options.experimental.enableStandaloneAppStartTracing = true
+}
+```
+
+```objc {tabTitle:Objective-C}
+@import Sentry;
+
+[SentrySDK startWithConfigureOptions:^(SentryOptions *options) {
+    options.dsn = @"___PUBLIC_DSN___";
+    options.experimental.enableStandaloneAppStartTracing = YES;
+}];
+```
+
+Since standalone app start transactions use the `app.start` operation, you can use a custom `tracesSampler` to set a dedicated sample rate for app starts without increasing your overall sample rate:
+
+```swift {tabTitle:Swift}
+import Sentry
+
+SentrySDK.start { options in
+    options.dsn = "___PUBLIC_DSN___"
+    options.experimental.enableStandaloneAppStartTracing = true
+    options.tracesSampler = { context in
+        if context.transactionContext.operation == "app.start" {
+            return 1.0
+        }
+        return 0.1
+    }
+}
+```
+
+```objc {tabTitle:Objective-C}
+@import Sentry;
+
+[SentrySDK startWithConfigureOptions:^(SentryOptions *options) {
+    options.dsn = @"___PUBLIC_DSN___";
+    options.experimental.enableStandaloneAppStartTracing = YES;
+    options.tracesSampler = ^NSNumber * _Nullable(SentrySamplingContext * context) {
+        if ([context.transactionContext.operation isEqualToString:@"app.start"]) {
+            return @1.0;
+        }
+        return @0.1;
+    };
+}];
+```
+
 ## Slow and Frozen Frames
 
 This feature is available for iOS, tvOS, and Mac Catalyst.
@@ -361,6 +427,7 @@ Starting with version 8.48.0, the Sentry SDK offers extensions for the types `Da
 The method signatures of these extensions resemble those of the original types, but they add a span to the current transaction. These methods can be used even when the Sentry SDK is not enabled—they will fall back to the original methods in that case.
 
 **Behavior:**
+
 - The SDK only creates spans when there is an active transaction on the scope
 - Only file URLs are tracked (non-file URLs like HTTP URLs are ignored, as they are handled by network tracing)
 - File size is automatically tracked for read and write operations