feat: hydrate durable Environment status

[sam2tom]

Summary

• add strict typed retrieval for the exact self-hosted Environment resource
• keep durable expired separate from live-only ready
• hydrate Environment only after the current Session supplies a valid ID
• integrate the read into buffered SSE recovery with Core, Session, Environment, request, event, epoch, selection, and abort fences
• keep canonical UUID identity consistent from an uppercase Session ID through Core's lowercase Environment response while opaque IDs remain exact
• degrade failed or malformed Environment reads without blocking conversation state
• update fixtures and protocol docs to Parsar 0438880a

Validation

Validated head: ba5016d7d68bfaa3c511ec23489391051eaf6e17

• pnpm 10.30.3 pnpm check: root 4/4, client 43/43, Web 155/155; typecheck and build passed
• full Playwright acceptance: 13/13 passed on isolated local ports
• canonical UUID browser regression: 5/5 repeated runs passed
• stream-rejected recovery still hydrates durable expired
• 503 and malformed resources degrade to Unavailable; newer live state wins
• every Environment request observed as a read-only GET after Session retrieval
• git diff --check and private-state ignore checks passed
• independent final diff review found no P0-P2 issues

The exact Environment lifecycle proof remains fixture-backed because the already-running local Core had no self-hosted Session during the earlier read-only inventory. No Turn, paid model call, deployment, merge, or branch/Worktree cleanup was performed.

Closes #16