fix(client): make the debug triple-tap gesture actually recognize three taps

[jeffrey701]

Problem

useKeyboardShortcuts (client/src/hooks/useKeyboardShortcuts.ts) registers a touch gesture documented as "Triple-tap (touch): Toggle debug panel (iPad/mobile)". The implementation is broken two ways:

if (e.touches.length !== REQUIRED_FINGERS) { tapCount = 0; return; }  // (1)
...
tapCount++;
if (tapCount >= 2) { tapCount = 0; toggleDebugPanel(); }              // (2)

1. It can never accumulate. A 3-finger tap arrives as separate touchstart events — 1 finger, then 2, then 3 (fingers never land on the same millisecond). The guard resets tapCount to 0 on each of those leading 1- and 2-finger events, so after a full 3-finger tap tapCount is only ever 1. On real hardware the panel never opens.
2. Wrong threshold. Even ignoring (1), it fires at tapCount >= 2 — a double-tap, not the documented triple-tap.

Fix

Extract a pure advanceTripleTap recognizer (client/src/hooks/tripleTap.ts): a touchstart that isn't exactly 3 fingers is ignored (not reset), only exact-3-finger touchstarts count, and the gesture completes on the third such tap within the 500ms window. The hook threads its state through the recognizer.

Tests

Adds tripleTap.test.ts: triggers only on the third 3-finger tap; the leading 1-/2-finger events of each tap don't reset progress (the real-hardware case); two taps don't trigger; an out-of-window tap restarts; state resets after a trigger. All five fail against the old logic (verified by swapping it back in) and pass with the recognizer.

Self-contained: only useKeyboardShortcuts.ts, a new pure recognizer, and its test; no engine/Rust/protocol changes.

Model: claude-opus-4-8

[gemini-code-assist]

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