Dashboard tile gestures: mixed drag/resize concurrency has no cross-gesture guard

Start with src/ui/dashboard-tile-gestures.ts, especially createTileGestureController, dragActive, installedGestureCancel, and the window pointer listeners. Read the concurrency characterization in tests/unit/dashboard.test.ts and the controller tests in tests/unit/dashboard-tile-gestures.test.ts. First resolve whether gestures should be mutually exclusive or intentionally concurrent; done means the chosen behavior is implemented and covered by tests, including disposal and pointer handling.

src/ui/dashboard-tile-gestures.ts's createTileGestureController has no cross-gesture mutual exclusion, despite the natural reading of "one gesture at a time":

• dragActive (declared line 191, guard at line 386 if (dragActive) return;) blocks only a second concurrent drag — a resize (wireGridResize) never checks it, so a resize can start and run while a drag is active, and vice versa.
• The single "currently cancellable gesture" slot, installedGestureCancel (declared line 200), is last-writer-wins and self-clearing: whichever gesture (re)installs it last (drag at line 659, resize at line 307) owns it; each gesture's own cleanup only nulls the slot if it is still the one holding it (drag clears at line 623, resize at line 280) — so an older gesture's cleanup never stomps a newer one's slot, but a dispose()/rerender only ever cancels whichever gesture currently holds the slot, leaving the other orphaned against soon-to-be-replaced DOM.
• Neither gesture filters its window pointermove/pointerup listeners by pointerId (module doc comment, lines 42-44) — a pointer other than the one that started the gesture still moves/ends it, and two concurrent resizes both terminate off one shared pointerup.

This is pinned by the "tile gesture concurrency characterization (#589 wave 2, CURRENT BEHAVIOR — not a guarantee, see inbox)" describe block in tests/unit/dashboard.test.ts and exercised at the controller-unit level across tests/unit/dashboard-tile-gestures.test.ts.

It is a pre-existing defect predating #589 (#593 refactor umbrella) — the extraction in wave 2 preserved it verbatim rather than introducing it. Fixing it is out of scope for a pure structural extraction; filing so a deliberate design decision (mutual exclusion vs. documented-intentional concurrency) can be made separately.