release: 2.1.6

[AminDhouib]

Version bump only: pyproject, README install snippets, uv.lock, CHANGELOG heading, regenerated RELEASE_CONTRACT.md.

2.1.6 ships the five fixes merged since 2.1.5, all found by rigorous testing of 2.1.5 on the live machine:

• F-868 (fix(F-868): status reports the backend this shell would use, not the first record; liveness ladder + adoption walk extracted to backend_liveness #104): status / doctor / stop / kill-orphans walk the adoption order instead of dict insertion order; restart reports the port it spawned on.
• F-869 (fix(F-869): get_instance_state reports storage truthfully; the storage reader no longer hashes nodriver's deep-serialized nodes #103): get_instance_state raises on a blocked storage read instead of reporting empty storage as the truth; messages never carry page values.
• F-872 (fix(F-872): cloner aspects return values, not serialization nodes; one JSON reader for every JS aspect; JS throws named from the exception #106): four cloner aspects return values, not nested BiDi transport nodes; a JS throw inside an extraction script is reported with its description.
• F-871 (fix(F-871): a named profile is no longer walked to -2 by a dead browser's leftovers, and the answer says when it is #105): a stale POSIX SingletonLock no longer walks a named profile to <name>-2.
• F-870 (ci(F-870): measure Chrome's cold start on every gate run; finding for the POSIX 'Failed to connect to browser' family #107): CI only, every gate run measures Chrome's cold start and uploads it as release evidence.

Merge LAST, then tag v2.1.6 on the merge sha.