Implements the fixes from the 2026-09-21 cutover dashboard read
(plans/sdk/2026-09-21-cutover-dashboard-fixes-plan.md):
- sdk_cutover_failed now carries `family` and `error_kind`
(network/conflict/http/sdk). The one production gate event was a raw
`TypeError: Failed to fetch` from an unwrapped fetch (readProjectFile /
writeProjectFile's preflight), which an instanceof-only classifier would
still file as "sdk" and trip the rollback gate — added a message-based
fallback for the browser's fetch-rejection strings.
- studio:sdk_session_unavailable's `network` reason now carries `elapsed_ms`
and `hidden` (document.visibilityState at rejection), separating a
near-instant policy block (CSP/PNA/extension) from a dropped connection
the tab or server going away mid-flight.
- The ownership-race branch in useSdkSession's open effect now emits
sdk_session_superseded instead of sdk_session_unavailable{stage:ownership}
— it's a benign race (identity moved on mid-open), not a broken
precondition, and was being counted as one on the dashboard.
- Root-caused and fixed the read-404 "storm": /api/events (CLI host) and
the vite-dev hot-reload channel are one connection per SERVER, not per
project. A stale tab left open after `hyperframes preview` restarts on a
different folder (same port) received every save the CURRENT project
made, reloading its preview (which then 404s) and re-reading its own
composition on each one. Both transports now carry `projectId`; the
client-side coordinator drops a delivery whose projectId doesn't match
the tab's (absent field still passes, for one release of skew).
- studio-server's file routes report `why` on 403/404: `project_dir_missing`
(new — the CLI host's static resolveProject kept answering for a project
whose folder was renamed/deleted under a running server, which used to
fail closed as an indistinguishable path-traversal 403) vs `nul` vs
`outside_project`.
- absent_or_empty reads now carry `path_in_tree` (against the caller's
loaded file tree). Instrumentation only, per the graveyard page
(clearing-activecomppath-on-tree-absence) that refuted the "clear
activeCompPath" fix this was the proposed next step for.
Tests: unit coverage added for every change (sdkCutover.test.ts,
useSdkSession.lifecycle.test.tsx, useExternalFileChangeCoordinator.test.tsx,
studioServer.test.ts, files.pathSafety.test.ts). oxlint/oxfmt clean,
tsc --noEmit clean on studio/studio-server/cli.
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
