test(harness): close the stale-assemble hole, and decide the dropped assertion

improvise-669b. PR #65 single-homed HANDSHAKE_TIMEOUT_MS in web/shell.js, but
the home is single in the TREE: the spec imports web/shell.js and the page
loads harness/serve/app/shell.js. createShell now exposes timeoutMs() beside
timesArmed(), and outlastTheWatchdog requires the running page's value to equal
the one it imported. Injected with the ticket's recipe (8000 -> 100 in source,
no rebuild): 5 passed / 1 failed where it was previously 6/6 green.

Also in scope, the same file's stale line: HANDSHAKE_TIMEOUT_MS restates
nothing and one test outlasts it, not two.

improvise-qf57. The formula-row click's `#grid input` toHaveCount(0) stays
dropped, and now deliberately. The reviewer's argument was that the exact
`#mode` assertion subsumes it; measuring gives a better one. app.js creates a
grid <input> in exactly one place, the grid cell's own mousedown, so no panel
handler can reach it — and an invented formula-row activation opens the
MINIBUFFER, not a grid editor. Injecting ChangeMode(formula_edit()) into
click_formula_row: the toHaveCount(0) probe PASSES and the run fails on the
#mode assertion below it. Restoring it would add an assertion that cannot
fail, which context/test-evidence/clients.md already records as the reason a
sibling assert! was removed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

web/shell.js

@@ -153,6 +153,9 @@ export function createShell({ onStatus, timeoutMs, setTimer = setTimeout, clearT
       this.setShell(text);
     },
     armed: () => watchdog !== null,
+    /** The window this shell was built with. Exposed so a spec can compare the
+     *  RUNNING page's value against the one it imported — improvise-669b. */
+    timeoutMs: () => timeoutMs,
     /** How many times the watchdog has been armed. A test denying its message
      *  needs this to be nonzero, or it is denying an unreachable event. */
     timesArmed: () => arms,