Plan / Interface: authoritative mecatui session reload

[jbeda]

Stage: Plan / Interface

Tracking: #2071. Batch 1 of 2; #2072 is blocked by #2071.

Defines server-authoritative mecatui session reload across reconnect, startup resume, and session adoption. A successful snapshot replaces server-derived title, mode, model/effort, context occupancy, usage, placement, state, and capabilities when supplied; local drafts, queues, and reading position remain view state. The plan specifies snapshot/live ordering and presence-aware legacy fallback. It does not implement early title generation.

The intended reader is an implementer/reviewer of the client reload contract. The owner for current client behavior is docs/tui.md; proposed behavior is kept in docs/acceptance/mecatui-authoritative-session-reload.md until implementation.

Review questions

• Is subscribe-before-snapshot plus bounded title-event buffering and run-result reconciliation sufficient for live/snapshot ordering?
• Are missing legacy fields distinguished from explicit empty/zero values correctly without widening the public wire API?
• Does snapshot authority on reconnect/adoption supersede only the existing title plan's snapshot-side revision gate?

Validation

• Acceptance-plan checker: passed (one advisory warning about named ADR/invariant tests; scenario proofs named).
• Acceptance-plan checker fixtures: passed.
• task docs: passed.

This PR is the human contract-review checkpoint. Implementation starts only after merge and a separate instruction.