Fix misleading host disconnect stop reason

[brsbl]

Human comments

What was wrong

The server's daemon-disconnect grace callback classified every unrecovered host/session socket loss as host-daemon-restarted, even when no new daemon instance had connected. That persisted reason flowed unchanged through the thread events API and timeline projection, so transient tunnel/session failures rendered as “Stopped — host daemon restarted.” A confirmed restart already has a separate stable signal: handleHostSessionOpened observes a changed daemon instanceId.

What changed

• Added the additive persisted interruption reason host-connection-lost and use it only when the daemon/session disconnect grace expires without a confirmed replacement instance.
• Preserved host-daemon-restarted for the existing changed-instanceId restart path.
• Added lifecycle failure copy and the timeline title “Stopped — connection to host was lost.”
• Added regressions at the server lifecycle boundary and thread-view parser boundary that prove lost connections and confirmed restarts remain distinguishable.
• The additive public reason changes the packed plugin SDK, so the established release guard requires the coherent patch bump 0.4.22 → 0.4.23. No host-daemon wire shape changed, HOST_DAEMON_PROTOCOL_VERSION is unchanged, and there are no CLI or documentation changes.

How you verified

• Red proof before the implementation: pnpm exec turbo run test --filter=@bb/server -- test/internal/background-task-reconciliation.test.ts failed 1/11 because the disconnect-grace event was host-daemon-restarted instead of host-connection-lost.
• Green focused regressions: the same server file passes 11/11; pnpm exec turbo run test --filter=@bb/thread-view -- test/parse-operation-message.test.ts passes 9/9.
• pnpm exec turbo run typecheck --filter=@bb/domain --filter=@bb/thread-view --filter=@bb/server --filter=@bb/app passes all 7 Turbo tasks.
• pnpm exec turbo run test --filter=@bb/server --filter=@bb/thread-view --filter=@bb/domain passes all 9 Turbo tasks, including 2,048/2,048 server tests (213 files pass, 1 corpus file skipped).
• node packages/plugin-sdk/scripts/check-npm-version-guard.mjs passes because @get-bb/plugin-sdk@0.4.23 is unpublished; focused SDK/domain typecheck and tests pass all 8 Turbo tasks (219 SDK and 172 domain tests).
• pnpm exec oxfmt --check on all six changed files and git diff --check pass.
• Exact final-head CI run 33030937107 passed all 13 jobs at 2d4894d97b0930905c9aca027b717c3558b01470, including checks, package/server/app/integration suites, Ubuntu and macOS package smoke tests, and Node 24/26 compatibility on Ubuntu and macOS.
• Exact branch web-app QA in Chrome for Testing 152.0.7977.64 passed at 390×844, 768×900, 1440×900, and 3440×1440. The real thread route passed initial render, client navigation, hard reload, responsive inspection, and console/runtime checks with the same deterministic fixture. The confirmed-restart row stayed unchanged while the disconnect row changed to the truthful text.

Before — PR #2472 head 79a9e0d273833790f75693d5ebb8b01a3fc4ed1e

Both the confirmed restart and unconfirmed disconnect render as a daemon restart.

After — child head 2d4894d97b0930905c9aca027b717c3558b01470

The confirmed restart remains unchanged; the unconfirmed disconnect is labeled as a lost host connection.

BB-Thread-ID: thr_sjdd7gudiq

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