fix(mobile): reconcile the two keyboard-dismiss paths (#279 + #280)

#279 and #280 auto-merge cleanly, but the merged result was red: neither
branch could see the other, and CI cannot see either, because the only test
covering #279 lives in test/mobile/** which test:ci excludes.

Two problems, both in #279's test:

1. The in-terminal case tapped the terminal's top-left corner, i.e. an inert
   transcript row, and asserted focus was retained. That is precisely the
   gesture #280 redefines, so #280 turned it red. Aim it at the PROMPT row
   instead: the one in-terminal tap whose outcome neither PR claims, so it
   still proves the #terminalContainer exemption without asserting the
   toggle's behaviour.

2. The "a real control is exempt" case was VACUOUS. It picked the first
   button measuring >8px, which is .welcome-ralph-link inside the welcome
   overlay hideWelcome() had already hidden: the rect still measures, but
   elementFromPoint at that point returns .xterm-screen, so the case tapped
   the TERMINAL and passed for the wrong reason. It only surfaced because
   #280 changed what a terminal tap does. Require the sampled point to
   actually resolve to the button, and fail loudly when no control is
   usable rather than silently asserting nothing.

Mutation-checked: removing the install, the #terminalContainer exemption,
the control exemption or the `if (moved) return` scroll guard each turns
the test red on its own. The control exemption had no coverage before.

Also fold the duplicated tap slop into one constant: initTerminal's
TAP_THRESHOLD now reads MOBILE_KEYBOARD_DISMISS_TAP_SLOP instead of
re-declaring 8, since a drift between them is exactly the bug the second
#279 commit fixed. And restore the comment the slop constant was inserted
into the middle of, which left "Regions where a tap must NOT dismiss"
sitting above the slop rather than the selector it documents.

test/mobile/keyboard.test.ts: 5 failed | 47 passed (52). Master is
5 failed | 46 passed (51) — the same five pre-existing failures.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/terminal-ui.js

@@ -32,13 +32,16 @@
   // short window, only the app's synthetic tap-to-position mouse event should
   // reach xterm.
   const TOUCH_COMPAT_MOUSE_SUPPRESS_MS = 450;
+  // Finger travel (px) still counted as a tap rather than a scroll. Shared by
+  // the terminal's own touch handling (TAP_THRESHOLD, initTerminal) and the
+  // keyboard-dismiss handler (_installMobileKeyboardDismiss), which MUST agree:
+  // a gesture the terminal treats as a scroll but the dismiss handler treats as
+  // a tap would close the keyboard mid-scroll and drop the composer.
+  const MOBILE_KEYBOARD_DISMISS_TAP_SLOP = 8;
   // Regions where a tap must NOT dismiss the on-screen keyboard
   // (_installMobileKeyboardDismiss). Two groups: anything that is about to take
   // focus itself, and the accessory bar, which is built to be used while the
   // keyboard is open.
-  // Finger travel (px) still counted as a tap for keyboard dismissal. Matches
-  // the terminal's own TAP_THRESHOLD so both agree on tap-vs-scroll.
-  const MOBILE_KEYBOARD_DISMISS_TAP_SLOP = 8;
   const MOBILE_KEYBOARD_DISMISS_EXEMPT_SELECTOR = [
     'input',
     'textarea',
@@ -675,7 +678,9 @@ Object.assign(CodemanApp.prototype, {
       let touchStartY = 0;
       let tapStartedWithTerminalFocus = false;
       let tapStartIntentCache = null;
-      const TAP_THRESHOLD = 8; // px — ignore micro-drift to distinguish tap from scroll
+      // px — ignore micro-drift to distinguish tap from scroll. Shared with the
+      // keyboard-dismiss handler so both classify the same gesture the same way.
+      const TAP_THRESHOLD = window.CodemanTerminalInput.MOBILE_KEYBOARD_DISMISS_TAP_SLOP;
       container.addEventListener(
         'touchstart',
         (ev) => {