Release v0.2.12 — session-file corruption worker-crash fix

Stability fix (high severity, DoS class):
Production-breaking TypeError on corrupted / empty / non-array
session payloads. unserialize() false → typed array assignment →
worker abnormal exit. Affects all v0.2.6 through v0.2.11.

Fixed at three call sites in src/Session/utils.php:
- zeal_session_start (request entry)
- zeal_session_reset (mid-request reset)
- zeal_session_decode (user-controlled input)

zeal_session_decode now returns bool matching PHP native signature.

11 new regression tests in SessionFileCorruptionTest.php cover
empty / corrupted / garbage / non-array / missing-file scenarios.

ROADMAP.md restructured around 'v0.2.x = security & migration'
framing. Connection pool moved out of v0.3 into remaining v0.2.x
items — it's a production-trust gap, not an observability feature.

204 unit tests, PHPStan green. See CHANGELOG.md and CRITIC.md.