fix: keep the Bun dev server correct and flat across hot reloads

[vivek7405]

Closes #1575

Found by a stress script that edits a running webjs dev app the way an AI agent does (bursts, partial writes, syntax errors then fixes, renames, deletes, atomic writes, a whole feature at once, checkout-like churn), against a fresh bun create webjs --db postgres app and Crisp's app.

Root causes (Bun, bun --hot)

• Every edit re-ran the CLI, so startServer ran again: a second handler, watchers, SSE hub, process handlers, instrumentation.register() and Bun.plugins, none disposed. ~25 MB per edit (153 MB to 5.8 GB over 225 edits), after which bun --hot stopped reloading and component edits were never served.
• A re-run built its route table before its watchers existed, so files changed while it booted were invisible to the instance that served (404s, unknown actions).
• Bun keeps a replaced file's old source forever (one atomic save and the page served its old version until restart), keeps a stale directory listing for a # import of a new file next to *.server.* modules, and hangs on re-importing a module that failed to parse.
• A *.server.* edit restarted the process (connection refused for 0.3 to 2 s); every dev re-import used a random query, leaking a module instance per request (~60 KB).
• Analysis caches restarted cold; the graph cache kept stale resolutions.

Fix

• dev/hot-host.js: the first run owns the process; a re-run (CLI fast path lib/dev-hot-rerun.js) only resets the registry and re-derives the analysis. Framework upgrade mid-session exits the child for the supervisor to restart.
• App modules load through a fresh-reading plugin (dev/bun-app-source.js), # aliases resolve through the app map (dev/bun-alias-resolve.js); the watcher pokes a temp-dir sentinel for a registry reset when needed, instead of restarting. The supervisor narrows restarts to instrumentation.* / env.* once the child announces in-place reload over IPC.
• devImport: content-keyed specifiers, never re-importing a failed one.
• Per-file caches for elision and the orphan scan; graph caches specifiers, not resolutions; watcher debounce 80 to 50 ms; routes.d.ts temp-name race fixed.

Results (stress script, 43 checks)

• Scaffold before: 4 fail, 13 slow, 5.8 GB after 225 edits. After: 43/43, p50 17 ms, max ~105 ms; RSS flat over 400 edits.
• Crisp before: 4 fail, 20 slow, p50 585 ms, max 2.2 s. After: 0 fail, p50 34 ms, max ~270 ms.
• Counterfactual: bun test/bun/dev-reload-stress.mjs against main's server/cli: 4 fail.

Tests

• npm test green (4973 pass); Bun matrix: all test/bun/dev-*.mjs green under Bun; new test/bun/dev-reload-stress.mjs (both runtimes), packages/server/test/dev/hot-reload-robustness.test.js, supervisor tests.
• Browser / e2e: N/A, no browser-side change (the reload frame protocol is unchanged).
• Docs: runtime reference, AGENTS.md, cli/server AGENTS, docs site runtime/ssr/websockets/configuration.

https://claude.ai/code/session_01SZ72LSPAo4NvYvBDSD6RLo

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