fix(desktop): defer timeline trim past StrictMode probe

[tlongwell-block]

Summary

• defer leave-time timeline trimming by one task so React StrictMode's synthetic setup → cleanup → setup cycle cannot trim an actively mounted channel
• cancel a pending trim when the same channel subscription sets up again
• keep the bounded-cache behavior on a real channel leave
• add focused scheduler regressions for cancellation, real leave, and channel-key isolation

Context

This is stacked on #1473. #1473 removes cap-driven eviction from active timeline merges; this follow-up makes its leave-time cleanup safe under the app's React.StrictMode mount probe.

This does not address the separate ancestor-island paging cursor bug. That contiguous-frontier fix is being developed separately and must compose with this patch before the missing-history freeze is lifted.

Validation

• desktop tests: 1497 passed, 0 failed
• desktop typecheck: clean
• pnpm check: clean
• full pre-commit hooks: clean
• pre-push hooks with the repo-pinned Hermit/Rust 1.95 toolchain: clean

An initial push attempt used Homebrew Rust 1.89 and correctly failed the buzz-db MSRV gate (sqlx 0.9 requires Rust 1.94). Re-running under the repository's pinned Hermit environment passed all hooks.