fix(reliability): review round 2 — boot-path logging, latch leaks, layout backup

[gyng]

Items 1–6 from the post-0.0.50 review:

1. No startup failure is console-only anymore — `useStudioInit`'s init catch (the choke point for every overlay init failure) and Canvas `reloadLayout`'s catch now route through `log_client` to the persistent log.
2. Recovery latch leak fixed — keepalive/displaywatch reset `RESPAWN_PENDING` when the main-thread dispatch itself fails; a stuck flag would have silently disabled recovery for the rest of the process.
3. displaywatch failures are visible — `RegisterClassW`/`CreateWindowExW` bails now log a warn (a dead watcher looked identical to a healthy one).
4. `log_client` input caps — message 4096 chars / component 64, char-safe truncation (pure seam + test), so a buggy render loop can't churn the 1 MiB log rotation.
5. Corrupt-layout backup — new `backup_layout` command copies `widgets.json` → `widgets.json.bad-` (newest 3 kept; pure prune seam + test) the moment the frontend fails to parse it, before the in-memory default layout can be saved over the original. Wired from both parse sites, once per session; read-failures (nothing to copy) skip it.
6. `recreateMain` retries once (2s) on a transient layout-read failure — studio-close is the only recreate attempt while secondaries keep windows alive, so a single hiccup used to strand the primary until the next layout edit.

Verified: `cargo test` 202 (2 new) / `clippy` / `fmt` clean; `tsc`, oxfmt+oxlint (0 warnings), 2769 unit tests, build.

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