refactor(map): drop unused exports in saved-map-authoring, dedupe extractionContext rebuild

Problem

Two cosmetic (non-blocking) findings from the Opus 5 confirming review of PR #350, deferred at merge time because neither affects behavior:

1. Unused exports in src/lib/map/saved-map-authoring.ts — getPackageRelevantMapConfig (line 47) and hasPackageRelevantMapConfigChanged (line 102) are exported but have zero consumers outside the module (verified: the only callers are internal, at lines 106 and 444). During the PR #350 nit round they gained an internal-only call site, so the export keyword is now dead surface area.

2. Redundant extractionContext rebuild in src/screens/MapScreen/MapScreen.tsx — extractionContext is built once via buildAuthoredLayerCommitContext (line 682) and consumed at lines 803/837, but validateAuthoredLayersForExtract (line 689) internally rebuilds an identical context object on every render. Cosmetic cost only.

Expected Behavior

• The two helpers are module-private (export keyword dropped) so the module's public API reflects real consumers.
• The duplicate context construction is either eliminated (share the built context) or documented as intentional.

Implementation Notes

• src/lib/map/saved-map-authoring.ts: remove export from the two functions. That's it — no caller changes needed.
• src/screens/MapScreen/MapScreen.tsx around lines 682–689: check validateAuthoredLayersForExtract's signature (exported from saved-map-authoring.ts) — either pass the existing extractionContext in, or have the validator accept the prebuilt context. If the validator's internal rebuild is deliberate (e.g. defensive copy), add a one-line comment instead.
• Pure refactor: no behavior change expected; tsc --noEmit + existing unit tests (tests/unit/lib/map/saved-map-authoring.test.ts, MapScreen suite) should pass untouched.
• Trivial diff, difficulty:small.

Acceptance Criteria

• getPackageRelevantMapConfig and hasPackageRelevantMapConfigChanged are no longer exported; grep -rn shows no external consumers
• No redundant per-render context rebuild in MapScreen, or a comment documents why it stays
• tsc --noEmit clean
• Focused unit tests pass (saved-map-authoring, MapScreen)

Filed from the Opus 5 confirming review of PR #350 (merge commit 20ea92d).

Start in src/lib/map/saved-map-authoring.ts and remove the exports from the two named helpers, then inspect src/screens/MapScreen/MapScreen.tsx around lines 682–689 for the duplicate context rebuild. Run tsc --noEmit and the saved-map-authoring and MapScreen tests; done means no external consumers remain and the rebuild is removed or documented.