feat(runtime): bind coordinator recovery generations

[briananderson1222]

Closes #195.

Summary

• add an explicit coordinator-facing recovery-fence writer that requires a pre-bound expected UUID generation
• atomically publish that exact generation under Flow's native mutation ticket
• retain ordinary writeRunRecoveryFence() behavior and its rejection of caller-supplied generations
• document the authority boundary and add runtime/type/export tests

Boundary

Flow remains provider-neutral and owns path safety, locking, atomic publication, inspection, and finalization. External coordinators remain responsible for authenticating and durably binding expected_generation before delegation. The generation is identity, not bearer authority.

Local verification

• npm test (352 Node tests passed; 26 browser tests passed, 8 intentional skips)
• npm run typecheck
• npm run check:types:contracts
• npm run docs:check
• npm exec --yes --package=@kontourai/veritas@1.5.2 -- veritas readiness (0 failures, 2 pre-existing warnings)

Hosted CI was intentionally skipped because the suite CI budget is exhausted. npm audit --omit=dev reports the two advisories already present on main (brace-expansion, fast-uri); this PR changes no dependencies.

[briananderson1222]

Local verification for exact head 56f88fecd878d7ee3f298418d5c84594f11705e1 is complete:

• full pre-push suite: 352 Node tests, console smoke, 26 browser tests passed with 8 intentional skips, then the 352-test schema lane passed again
• typecheck, public type contracts, docs checks, and Veritas readiness passed (0 failures; 2 pre-existing warnings)
• independent code review: clean after required-generation regression fix
• independent security review: clean
• downstream Flow Agents coordinator suite: 69/69 passed against an exact tarball of this commit

The PR remains blocked only because the three protected GitHub Actions contexts are absent while hosted CI is intentionally skipped for the exhausted CI budget. No required context was synthesized and branch protection was not changed.