feat: Add per-class identityMode via @js macro parameter

Add identityMode: Bool parameter to @js macro. When set to true on a class,
that class uses pointer identity caching. When false or not set, the class
uses the bridge-js.config.json default.

Identity is resolved entirely at codegen time - no runtime option. Classes
with identity mode get static __identityCache passed to __wrap. Classes
without it pass null. No runtime branching in __construct.

Resolution: @js(identityMode: true/false) > bridge-js.config.json > default (off).

Author: krodak

Benchmarks/README.md

@@ -38,15 +38,35 @@ node run.js --filter=/string/i
 
 ## Identity Mode Benchmarks
 
-Compare `identityMode: "pointer"` vs default (`"none"`) for SwiftHeapObject wrapper caching. Requires `--expose-gc` for memory benchmarks.
+Compare `identityMode: "pointer"` vs default (`"none"`) for SwiftHeapObject wrapper caching. Both class variants are compiled into the **same build** via per-class `@JS(identityMode: true)` annotations, so no separate builds or config files are needed.
+
+Requires `--expose-gc` for memory benchmarks.
+
+### Build once
+
+```bash
+swift package --swift-sdk $SWIFT_SDK_ID js -c release
+```
+
+### Compare both modes in one run
 
 ```bash
-# Run identity benchmarks comparing both modes
-node --expose-gc run.js --identity-mode=both
+node --expose-gc run.js --identity-mode=both --identity-iterations=1000000
+```
+
+### Run only one mode
 
-# Pointer mode only
-node --expose-gc run.js --identity-mode=pointer
+```bash
+# Only pointer (identity-cached) classes
+node --expose-gc run.js --identity-mode=pointer --identity-iterations=1000000
 
+# Only non-identity classes
+node --expose-gc run.js --identity-mode=none --identity-iterations=1000000
+```
+
+### Additional options
+
+```bash
 # Control iteration count (default: 1000000)
 node --expose-gc run.js --identity-mode=both --identity-iterations=500000
 
@@ -59,8 +79,8 @@ node --expose-gc run.js --identity-mode=both --identity-memory
 # Combine with adaptive sampling
 node --expose-gc run.js --identity-mode=both --adaptive
 
-# Save identity results
-node --expose-gc run.js --identity-mode=both --output=results/identity-mode/results.json
+# Filter to specific identity benchmarks
+node --expose-gc run.js --adaptive --filter=passBothWaysRoundtrip --identity-mode=both --identity-iterations=1000000
 ```
 
 ### Identity Mode Scenarios

Benchmarks/Sources/Benchmarks.swift

@@ -305,6 +305,61 @@ nonisolated(unsafe) var _cachedPool: [SimpleClass] = []
     }
 }
 
+// MARK: - Identity Mode Benchmark Variants
+// These classes use @JS(identityMode: true) so that identity cache benchmarks
+// can run in the SAME build alongside the non-identity classes above.
+
+@JS(identityMode: true)
+class SimpleClassIdentity {
+    @JS var name: String
+    @JS var count: Int
+    @JS var flag: Bool
+    @JS var rate: Float
+    @JS var precise: Double
+
+    @JS init(name: String, count: Int, flag: Bool, rate: Float, precise: Double) {
+        self.name = name
+        self.count = count
+        self.flag = flag
+        self.rate = rate
+        self.precise = precise
+    }
+}
+
+@JS(identityMode: true)
+class ClassRoundtripIdentity {
+    @JS init() {}
+
+    @JS func roundtripSimpleClassIdentity(_ obj: SimpleClassIdentity) -> SimpleClassIdentity {
+        return obj
+    }
+
+    @JS func makeSimpleClassIdentity() -> SimpleClassIdentity {
+        return SimpleClassIdentity(name: "Hello", count: 42, flag: true, rate: 0.5, precise: 3.14159)
+    }
+
+    @JS func takeSimpleClassIdentity(_ obj: SimpleClassIdentity) {
+        // consume without returning
+    }
+}
+
+nonisolated(unsafe) var _cachedPoolIdentity: [SimpleClassIdentity] = []
+
+@JS(identityMode: true)
+class IdentityCacheBenchmarkIdentity {
+    @JS init() {}
+
+    @JS func setupPool(_ count: Int) {
+        _cachedPoolIdentity = (0..<count).map {
+            SimpleClassIdentity(name: "Item \($0)", count: $0, flag: true, rate: 0.5, precise: 3.14)
+        }
+    }
+
+    @JS func getPoolRepeated() -> [SimpleClassIdentity] {
+        return _cachedPoolIdentity
+    }
+}
+
 // MARK: - Array Performance Tests
 
 @JS struct Point {