Split the image snapshots into one bundle per module

[kyleve]

Gives each module its own image-snapshot bundle — WhereUISnapshotTests, PeriscopeToolsSnapshotTests, SwiftDataInspectorSnapshotTests — all listed in the single StuffSnapshotTests scheme, so CI still runs them in one invocation.

Stacked on #145. Review/merge that first.

This corrects my mistake in #143

I argued there for a single shared bundle, on the grounds that a second one would be unsafe: several .xctest bundles in one scheme load into one StuffTestHost process, where each bundle's own copy of SnapshotKitTesting's capture state (the safe-area swizzle's depth counter, SnapshotCaptureLock, the UIView category) would fight over the single shared UIView method exchange.

The premise is false. I inherited "one scheme means one process" from the root AGENTS.md double-linking note instead of measuring it. Probing ProcessInfo.processIdentifier from two bundles in one scheme:

full unfiltered Stuff-iOS-Tests run:
  SnapshotKitTestingTests  pid=12076
  WhereUITests             pid=12146

Different PIDs, on both a filtered and a full run. xcodebuild gives each test bundle its own host process, so each copy of that state is genuinely process-wide — exactly as its documentation claims. The hazard is real, but it needs co-loading, which doesn't happen.

Splitting is better than merely permissible

Each module's bundle now links only what that module needs. PeriscopeToolsSnapshotTests depends on PeriscopeTools and SwiftDataInspectorSnapshotTests on SwiftDataInspector — neither builds against WhereUI any more. That's the layering wart I explicitly accepted when arguing for the shared bundle, now gone.

References were already per-module (swift-snapshot-testing derives __Snapshots__ from the caller's #filePath), so no image moved and none were re-recorded.

Docs

The "never add a second image-snapshot bundle" rule is replaced with the measurement and the assumption it rests on: if a toolchain ever starts sharing one host process across bundles, re-measure before adding another. Updated across the root AGENTS.md, SnapshotKitTesting, and the three modules' AGENTS.md/README.md.

The #143 completed TODO keeps its wrong reasoning with a correction appended rather than being edited away. Two load-bearing claims in these docs have now turned out to be inherited rather than verified — the "WhereUI is a dynamic framework" one in #145, and this — and that pattern seems worth leaving visible.

Note on #145

Its filed spike item existed to make a second snapshot bundle safe. That motivation is now moot, so this PR reframes the entry: the findings stay recorded (they're still accurate about how dynamic linking behaves), but it's explicit that the reason for wanting it evaporated.

Verification

• All three bundles green from the one scheme, with three separate All tests boundaries in the output — the process separation showing up directly.
• Zero reference images modified.
• Stuff-iOS-Tests green.
• ./swiftformat --lint clean.