Add opt-in inconsistent_default_argument_in_file rule

[nenadvulic]

Summary

Adds a new opt-in rule inconsistent_default_argument_in_file (kind: lint), as discussed in #6668.

It flags overloaded functions declared on the same type — the type's primary declaration and any extensions of it in the same file — when a shared (label, type) parameter uses different default values. This catches accidental default drift across overloads that callers reasonably expect to behave the same, e.g.:

struct Network {
  func fetch(_ url: URL, retries: Int = 3) {}
  func fetch(_ url: URL, headers: [String: String], retries: Int = 0) {} // ← silently drops to 0
}

Scope

Per the agreement with @SimplyDanny and @rgoldberg in #6668, this is the intentionally small, purely syntactic, single-file variant:

• Opt-in (disabled by default) — teams that want the "overloads share defaults" invariant get it; others aren't bothered.
• Behavior = across_one_type: a type's primary declaration plus all extensions of it in the same file are treated as one scope (matched via the fully-qualified lexical type name, e.g. Foo.Bar).
• Nested types form their own scope (separately): Parent.f(…) and Parent.Nested.f(…) are functions on different types, so they are never compared — comparing them would produce false positives.
• The offending parameter is the one whose default diverges from the first declaration of that shared parameter, in source order.

Deliberately out of scope here (left to a potential future analyzer rule, so no name clash):

• call_default_arguments and override_default_arguments — both need type/symbol resolution unavailable to a syntactic rule.
• Cross-file cases.

The _in_file suffix keeps the name honest about the limitation and leaves room for a fully-general analyzer rule later. Config options floated in the discussion (container_scope, nested_containers, parameter_grouping, …) are intentionally not included yet; they can be layered on additively without breaking the v1 defaults.

Examples

Triggering:

struct S {
  func f(x: Int = 1) {}
  func f(↓x: Int = 2, y: Int) {}
}

struct S {
  func f(x: Int = 1) {}
}
extension S {
  func f(↓x: Int = 2, y: Int) {}
}

Non-triggering (same default, one-sided default, different type, different scope, nested type):

struct S {
  func f(x: Int = 1) {}
  func f(x: Int = 1, y: Int) {}
}
struct Outer {
  func f(x: Int = 1) {}
  struct Inner {
    func f(x: Int = 2) {} // different scope
  }
}

Closes #6668.

[SwiftLintBot]

	12 Warnings
⚠️	This PR introduced a violation in DuckDuckGo: /iOS/AutofillCredentialProvider/CredentialProvider/Shared/ActionMessageView.swift:112:25: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in DuckDuckGo: /iOS/AutofillCredentialProvider/CredentialProvider/Shared/ActionMessageView.swift:132:33: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in DuckDuckGo: /iOS/Core/Pixel.swift:295:29: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in DuckDuckGo: /SharedPackages/BrowserServicesKit/Sources/BrowserServicesKit/Email/EmailManager.swift:721:52: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in DuckDuckGo: /macOS/LocalPackages/BuildToolPlugins/Plugins/InputFilesChecker/Errors.swift:148:9: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in Firefox: /firefox-ios/Client/Extensions/UIView+Screenshot.swift:43:69: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in Realm: /RealmSwift/Tests/KVOTests.swift:193:9: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in Realm: /RealmSwift/Tests/KVOTests.swift:214:9: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in Realm: /RealmSwift/Tests/ProjectionTests.swift:734:35: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in Realm: /RealmSwift/Tests/ProjectionTests.swift:760:35: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in Realm: /RealmSwift/Tests/ProjectionTests.swift:786:35: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)
⚠️	This PR introduced a violation in WordPress: /Modules/Sources/WordPressData/Swift/WordPressComRestApi+Defaults.swift:18:41: Warning: Overloaded functions declared on the same type (its primary declaration and any extensions of it in the same file) should not use different default values for a shared parameter, as this is usually an accidental drift that callers do not expect (inconsistent_default_argument_in_file)

	19 Messages
📖	Building this branch resulted in a binary size of 28242.07 KiB vs 28200.24 KiB when built on main (0% larger).
📖	Linting Aerial with this PR took 0.64 s vs 0.65 s on main (1% faster).
📖	Linting Alamofire with this PR took 0.94 s vs 0.96 s on main (2% faster).
📖	Linting Brave with this PR took 6.03 s vs 6.07 s on main (0% faster).
📖	Linting DuckDuckGo with this PR took 26.04 s vs 25.93 s on main (0% slower).
📖	Linting Firefox with this PR took 10.44 s vs 10.44 s on main (0% slower).
📖	Linting Kickstarter with this PR took 7.44 s vs 7.43 s on main (0% slower).
📖	Linting Moya with this PR took 0.35 s vs 0.35 s on main (0% slower).
📖	Linting NetNewsWire with this PR took 2.34 s vs 2.35 s on main (0% faster).
📖	Linting Nimble with this PR took 0.54 s vs 0.53 s on main (1% slower).
📖	Linting PocketCasts with this PR took 6.97 s vs 6.89 s on main (1% slower).
📖	Linting Quick with this PR took 0.37 s vs 0.35 s on main (5% slower).
📖	Linting Realm with this PR took 2.45 s vs 2.45 s on main (0% slower).
📖	Linting Sourcery with this PR took 1.49 s vs 1.49 s on main (0% slower).
📖	Linting Swift with this PR took 4.29 s vs 4.29 s on main (0% slower).
📖	Linting SwiftLintPerformanceTests with this PR took 0.16 s vs 0.16 s on main (0% slower).
📖	Linting VLC with this PR took 1.09 s vs 1.08 s on main (0% slower).
📖	Linting Wire with this PR took 15.39 s vs 15.33 s on main (0% slower).
📖	Linting WordPress with this PR took 9.9 s vs 9.93 s on main (0% faster).

Generated by 🚫 Danger

[nenadvulic]

@SimplyDanny the rule behaves exactly as scoped in this thread 6668 : grouping by (label, type) within a type's declaration + its extensions in the same file, nested types isolated.

All matches are intra-type (no cross-type false matches). The mix of real catches and intentional differences is expected and is the reason for the opt-in default; teams that want the "overloads share defaults" invariant enable it, others aren't affected.