app: v3 Preview Debug Status shows the session's counters (row C25)

[GoKubar]

Sub-lane of P5-APP-PARITY-FINISH, app-parity row C25 (Preview Debug Status, developer-facing).

Under v3, View > Show Preview Debug Status showed only session.environmentNote (and only when ready). The v2 pane's strip (PreviewV2View.swift ~:1052) showed the frame's id, project, revision, pages, window and fonts.

• EngineV3Session.debugLine (pure, computed): host pid or "no host", host starts, last DONE id (cancelled DONEs excluded, as lastDoneID is), layout revision, pages and stale pages, DONEs received, keystroke-to-screen median over the last 200 samples (same computation as the status bar in ShellChrome), environment note. Only fields the session already keeps; nothing new is tracked.
• PreviewV3StatusHUD shows it whenever the toggle is on (any phase), id engine-v3.debug-status.
• EngineV3DebugStatusTests: testTheDebugLineCarriesTheSessionsCounters (FLASHTEX_HOST=none, private FLASHTEX_V3_CACHE, no bundle; feeds STARTED/PAGES/DONE and a cancelled DONE) and testTheDebugLineShowsTheKeystrokeMedian (latency samples on a bare session). Neither needs a host, so both run on hosted mac-app.

Not done here: the v2 strip's font manifest has no v3 equivalent in the session; it is not invented.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SDAiBvi7bfkb19jLNPTtjp

[GoKubar]

CI run 37505201540: green. In the mac app job (112412041399), the full log (artifact mac-swift-test-log) shows that EngineV3DebugStatusTests.testTheDebugLineCarriesTheSessionsCounters and testTheDebugLineShowsTheKeystrokeMedian were executed and passed, not skipped. Totals: "Executed 2401 tests, with 132 tests skipped and 0 failures".

[GoKubar]

Independent review (kabir-claude reviewer, P5-APP-PARITY-FINISH): approve. Reviewed at c194967. I only read code and CI.

• Row C25. Under v3 the toggle used to show only environmentNote, and only when ready. It now shows EngineV3Session.debugLine: host pid and starts, last DONE, layout revision, pages and stale pages, DONEs received, the keystroke median and the environment note. That is the v3 equivalent of v2's strip (frame id, revision, pages). The change is in the app only; no engine code.
• Tests. EngineV3DebugStatusTests drives handle(.started/.pages/.done) with FLASHTEX_HOST=none. It checks the exact line, stale counting, that a cancelled DONE does not become "last DONE", and the median over the last 200 samples. Neither test has a skip path, so they run on the hosted mac-app job.
• CI (run 37505201540). mac-app passed: 2401 executed, 0 failures.
• Nits. With an even sample count the median is the upper median. The line relies on @ObservationIgnored counters changing together with observed state (the comment says so). That is fine for a developer-only strip.