feat(lockfile): switch drift.lock to versioned TOML array-of-tables format

[laulauland]

Summary

drift.lock now uses a versioned TOML format:

version = 1

[[bindings]]
doc = "docs/auth.md"
target = "src/auth/login.ts"
sig = "a1b2c3d4e5f6a7b8"

The new format reduces spurious merge conflicts on disjoint edits (each binding is a separate block with its own metadata) and adds explicit lockfile versioning for future migrations.

Compatibility

• Existing pre-TOML lockfiles are still readable: drift check, drift status, and drift refs parse the legacy line-based format unchanged.
• The next mutating command (drift link, drift unlink) rewrites drift.lock to version = 1 TOML.
• After that rewrite, older drift binaries can no longer read the lockfile. Teams pinning drift in CI or dev environments should upgrade all installations together before running commands that modify drift.lock.

Doc updates

• README.md — TOML examples for the lockfile and origin-qualified anchors
• docs/DESIGN.md — TOML data model, on-disk format section, and lockfile.zig description
• docs/DECISIONS.md — new §11 covering format, canonical sort, and forward-compat parser rules
• .claude/skills/drift/SKILL.md — TOML anchor and origin examples
• docs/CLI.md — drift link description references the TOML lockfile

Test plan

• zig build test — all 293 tests pass, including new TOML parser/serializer tests, version handling, and the legacy-line-format compatibility test
• drift check clean on this repo's own (already-rewritten) drift.lock
• Property tests in the merge oracle confirmed the format substantially reduces spurious merge conflicts vs. the line format