fix(location): unify watch handles across mobile surfaces

Closes #220

Summary

• make the flat craft.watchPosition and craft.clearWatch surface delegate to the typed location implementation and return numeric watch handles
• share one native location stream across typed and legacy subscribers without allowing either API to clear the other's active watches
• remove rejected-start callbacks, allow a later retry, and prevent rejected native starts from becoming unhandled promise rejections
• preserve no-argument clearWatch() compatibility by clearing every active watch with at most one native clear request
• update the public TypeScript declarations and lock the bridge surface with Zig tests

Regression proof

The new tests were run against the pre-fix implementation first. Five page-script cases and the bridge-surface assertion failed, covering numeric flat handles, shared-stream ownership, rejected-start cleanup/retry, listener cleanup, and no-argument clearing.

Explicit mutation checks also proved that the suite rejects:

1. restoring the flat geolocation delegation
2. removing rejected-start cleanup
3. retaining callbacks after no-argument clear
4. letting a legacy clear stop an active typed watch

Verification

• bun test in packages/ios: 73 passed
• zig build test:ios: passed
• bun run test:sdk: 566 passed
• bun run typecheck: passed
• bun run verify:packages: 36 checks passed
• bunx --bun pickier .: 0 errors (36 existing warnings)
• git diff --check: passed
• zig fmt --check packages/zig/test/bridge_surface_test.zig: passed
• iOS template compilation reached and passed the iPhone, Live Activity, Watch extension, and Watch app compile stages; the final embedded Watch-app scheme cannot run locally because this host does not have the watchOS 26.5 runtime installed

The repository-wide Zig suite remains blocked locally by the existing external sibling zig-js incompatibility with the installed Zig 0.17 development toolchain. The focused iOS target passes, and CI uses the repository-pinned toolchain.