CAS: keep the mount lease with one thread and one renewal path

[filimonov]

One thread per writable CAS mount now renews the lease and reclaims the slot in turn. The two-worker parking protocol (nine driver states), the bounded re-anchor renewals and the thread-local observability stack are removed, with no change of guarantees. Lease threads per mount: 2 → 1; driver states: 9 → 0; renewal entry points: 4 → 1; production code: −624 lines.

Stacked on the fix branch fix/antalya-26.6/cas-mount-renewal-no-deadline so that the diff shows this change only; to be retargeted to antalya-26.6 after that branch merges.

Related: #2474
Related: #2431

Changes

• One lease thread. A reclaim starts with the fence latched and arms it only under one rule: no remount request pending, no stop, the lifecycle not terminal, the deadline admits a ref-log append; the check and the arm are one step under one mutex.
• Readiness goes through the same loop: an open returns, and a remount reports Live, only when a write can be admitted. A claim that is too old leaves the fence latched and the next renewal arms it.
• One renewal path. The startup and remount re-anchors, the lease-bound renewal policy and the fence-gated renewal plane are gone.
• The renewal returns what its report needs; events and log lines are byte-compatible.
• An interference report trips the fence and requests the remount in one step. Before, a reclaim that finished between the two steps re-opened the fence.
• Twelve small cleanups: one definition per duplicated quantity, removed write-only state.

Risks / notes

• Removed: the event CASMountRenewalDeadlineExceeded and the classifications external_lease_deadline and request_deadline; no renewal can end at a deadline any more.
• A writable open with no lease thread whose claim cannot admit a write now fails with a retryable error. No production path opens that way.
• A lease thread that leaves through its error path keeps the mount closed until restart. Before, the mount flapped. Documented in troubleshooting.md.
• A remount step claimed_not_armed is reported for a reclaim that claimed and could not arm; it counts in CASRemountSucceeded.

Testing

• Unit: CAS* gate, 2613 tests in release, 2617 under ASan with no error.
• Integration: test_cas_mount_renewal_retry, 6 tests.
• Each rule of the lease keeper has a named test.

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

The CAS mount lease is kept by one thread that renews and reclaims in turn. A mount is reported Live only when a write can be admitted. Remove the event CASMountRenewalDeadlineExceeded and the renewal classifications external_lease_deadline and request_deadline, which can no longer occur. Fix an interference report that could leave the mount fence open until the next remount.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

Updated docs/en/antalya/cas/architecture/mounts-and-leases.md, docs/en/antalya/cas/configuration.md, docs/en/antalya/cas/operations/troubleshooting.md, docs/en/antalya/cas/operations/monitoring.md and docs/en/antalya/cas/operations/debugging.md.

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Unit tests
• Performance tests
• Aarch64 tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• CAS (content-addressed storage; Antalya only)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• OAuth (5m)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• S3 Export (2h)
• Swarms (30m)
• Tiered Storage (2h)