What Problem This Solves

The private iMessage plugin still exported twenty-five helper types, constants,
and one normalization function that are only referenced inside their defining
modules. Those exports enlarge the apparent implementation API and add noise to
dead-code analysis.

Why This Change Was Made

Localize the remaining declarations while preserving the exports intentionally
exposed by extensions/imessage/api.ts. This is a module-surface cleanup only;
definitions, callers, values, and control flow are unchanged.

User Impact

No user-visible behavior change. The bundled private plugin exposes a smaller,
more accurate internal module surface.

Evidence

• Fresh full, non-shallow worktree from origin/main at 81d0f3f37291b199005ace7c55db1d9c6d060c70
• codebase-memory graph refreshed on commit 765c1b36e98d3b21fe0f3abbee1098cc7b52b8d4: 21,043 files, 281,325 nodes, 1,139,718 edges
• repository-wide source scan confirms all twenty-five declarations are confined to their defining modules
• post-edit scanner reports zero remaining iMessage candidates in this batch
• Testbox tbx_01kwzk7wy9zabpb7dsy18mxyvk: pnpm check:changed
• Testbox tbx_01kwzk7wy9zabpb7dsy18mxyvk: iMessage suite, 60 files / 726 tests passed
• Testbox tbx_01kwzk7wy9zabpb7dsy18mxyvk: full pnpm build
• fresh local autoreview: clean, confidence 0.89
• fresh branch autoreview: clean, confidence 0.86
• git diff --check