perf(desktop): make warm navigation paint immediately

[wesbillman]

Summary

• paint warm channel and thread content from retained cache while revalidation runs behind it
• bound retained timeline work and preserve virtualizer/provenance behavior across warm navigation
• immediately replace the old channel pane with the destination skeleton before expensive cached rows mount
• cancel superseded deferred navigation, including back/forward, same-channel reselects, and failed huddle routes
• add unit, E2E, and performance coverage for warm switching and navigation lifecycle ordering

Validation

• pre-push desktop check
• pre-push desktop typecheck
• pre-push desktop unit suite: 4,959 passed
• focused navigation regression E2E coverage
• independent review cleared exact head 448587bc2db55125bff355508b60f48ba3fdaad6

Residual risk

• a millisecond-scale supersede window remains after route navigation begins but before commit; fully closing it requires cancellable navigation in commitNavigation
• forced rejected-navigation behavior is code-reviewed rather than E2E-injected

[themiguelamador]

I found two correctness regressions in the warm-navigation path and fixed both on Complear/buzz:review/pr-6069-fix (002fd6831).

1. The fresh-cache shortcut skipped the mandatory page-to-live catch-up when returning to a channel. The global listener only updated the projected flat cache, so a later authoritative-window projection could erase messages received while the channel was away; thread summaries could be missed as well. The subscription now always performs one background head catch-up, with a regression test that receives a message while away, returns, receives another live message, and verifies both remain.

2. Deferred sidebar navigation read pendingChannelId from React render state. Two clicks in the same task (another channel, then the current channel) therefore let the first deferred navigation survive because the second handler still saw null. A synchronously maintained ref now owns that cancellation decision; the PR's existing reselect/cancellation E2E passes.

Validation: Desktop lint/file-size check and TypeScript typecheck pass; the complete Desktop unit suite passes (4,959 tests); the E2E build passes; all previously run focused navigation cases passed except the reproduced reselect failure, and after the fix both the reselect case and the new away-message regression pass.

Please incorporate the fix commit (or equivalent changes) before merging.