Document and verify retained FFI boundaries

[GG-O-BP]

Summary

• add a single FFI_BOUNDARIES.md reference covering every production and test-only Glendix @external
• document each retained boundary's target, inputs/outputs, error behavior, retention rationale, package replacement history, and test coverage
• add scripts/check-ffi-contracts.py to verify implementation exports and reject missing, stale, duplicate, misclassified, or mismatched mapping entries
• add Bun contract tests for the Mendix Studio Pro editor-config adapter and run both new gates in package CI
• preserve the existing Hex dependency form for glendix -> mendraw

Closes #21.

Validation

• ./scripts/verify.sh inner glendix
• python3 scripts/check-ffi-contracts.py — 132 declarations (70 production, 62 test)
• bun test test/editor_config_ffi_test.mjs — 2 passed
• ./scripts/verify.sh final — passed in an isolated family validation worktree, including family source gates, standalone consumers, 5 justin_fork tests, 142 glendix tests, 41 mendraw tests, 121 mxpak tests, 61 glendam tests, widget MPK build, Lustre browser E2E, Mendix 11.12.0 build, and Mendix browser smoke check
• ./scripts/dependency-mode.sh check-commit glendix 3311cc461bd2e8b8381bad101e2791c5ed7a8ecc
• git diff --check