fix(desktop): separate relay readiness from background sync

[wesbillman]

Pinky, an AI agent, is opening this PR on Wes's behalf.

Summary

Separate authenticated relay readiness from background history synchronization, and remove a redundant per-channel subscription family from Desktop startup.

• Derive Home mention signals from the existing member-channel live streams instead of opening a second #p subscription per channel. Keep mention kinds, recipient/self/member policy, deduplication, latest callbacks/identity, and single-channel context for untagged auxiliary events.
• Resolve connection readiness after AUTH rather than after reconnect replay/history repair. Start the watchdog and publish reconnect readiness while repair continues in the background.
• Fence background replay failure recovery to its connection generation. New live subscriptions still wait for the shared admission gate and reject if superseded while waiting.
• Snapshot reconnect subscriptions before any gate wait so a newly created subscription cannot also be replayed when a throttled timer fires late.

Scope: this is the first structural slice, not a claim that the reported startup send stall is fully fixed. Cold-start fan-out remains unpaced, both native relay sessions share the relay's per-principal quota, and exhausted/ambiguous send recovery remains separate work. This does not exempt user sends from authoritative relay backoff.

Related work and stack

• Stacked on fix(relay): reject a frame on its own acknowledgement channel #6961 (brain/diagnose-send-stall at 3b06dd32493596ec650f20abf8805791c50fdc24). Only one new commit and six Desktop files are in this review range: 49e57ce5557d5830b4adedb46e56b82f22196515.
• Do not merge this into the parent feature branch. After fix(relay): reject a frame on its own acknowledgement channel #6961 lands, rebase/retarget this PR onto main while preserving this six-file scope.
• fix(desktop): bound HTTP event submission #6957 is the separate HTTP deadline/recovery investigation, not included here. Open startup-related issues/PRs were searched; fix(relay): reject a frame on its own acknowledgement channel #6961 and fix(desktop): bound HTTP event submission #6957 are the closest related work.
• Originating conversation: buzz://message?channel=5b130804-d759-40ad-a564-d64cc907fa8e&id=0280e4c9a91a1921b08e299b16b3ee12aea87e3a81b09d66752605b895ea2b95&thread=da4de2751cf62feda9e9d3869e5af3201fa6a9fe91407f7f5430a759498ed6ea

Testing

At pushed head 49e57ce5557d5830b4adedb46e56b82f22196515:

• All nine authoritative pre-push lanes passed: push-head-scope, branch-skew, file-size-check, desktop-check, rust-tests, desktop-typecheck, mobile-checks, desktop-test, and desktop-tauri-checks. Pre-commit formatting/lint passed too.
• 10 mounted React-hook regressions count 20/50/129 member-channel streams (not 40/100/258), verify the remaining wire filter covers every Home mention kind, and exercise replay dedupe, notification boundaries, membership/callback/identity updates, auxiliary routing, setup retry, and cleanup.
• 8 real RelayClient IPC-dispatch regressions cover held repair with a successful plain send, AUTH gating/rejection, stale/current replay failures, backoff admission, community replacement, and delayed-timer replay snapshot ordering. Only the Tauri transport/repair boundary is mocked.
• Eight deliberate mutations were killed: restoring await-history, removing stale-replay fencing, bypassing admission, removing admission generation validation, delaying the replay snapshot, and removing mention recipient/dedupe/wire-kind guards.
• Read-only independent review completed. Unpaced fresh subscriptions remain a follow-up; a suggested EVENT backoff exemption was rejected because the server quota is shared. The unconditional gate await is deliberately generation-guarded.

Not verified: native before/after send latency, a packaged composer click-through, and retained operation identity after exhausted/ambiguous failure. Native capture is paused. No visual UI changes or screenshots in this slice. GitHub CI will validate this exact stacked range separately from the local gates.