fix: viewer affordance for text/blob/json and overflow-detected cells (#216)

[EVWorth]

What

Fixes three compounding bugs in TruncatedCell:

1. Stale slice. Cell text was sliced to 47 chars + "..." in JS at render time, so dragging a column wider never revealed more of the value.
2. No affordance for short structured values. TEXT/BLOB/JSON/MEDIUMTEXT/LONGTEXT columns got no viewer hint, even though the viewer (formatting, search, character count) is useful regardless of length.
3. Heuristic ≠ reality. isLongValue = length > 50 was used to decide truncation visibility — didn't match actual visual overflow.

Why

The first is the literal #216 report. The second and third are downstream consequences of the same heuristic that the bug exposed.

Changes

File	Change
src/components/grid/TruncatedCell.tsx	Render full formatValue(value) via CSS truncate. Add ResizeObserver-driven isOverflowing state + rAF debounce. Render a Maximize2 icon button when showForTextType (text/blob/json type AND ≥ 20 chars) OR isOverflowing. Add optional dataType prop. Icon click opens viewer and stops propagation. Double-click kept as fallback.
src/components/grid/ResultsGrid.tsx	Pass col.data_type as the new dataType prop.
src/components/grid/__tests__/TruncatedCell.test.tsx	Rewrite as 19 cases: NULL handling, primitive types, full-text render, no-icon for short non-text, icon appearance for each text/blob/json variant, ResizeObserver-driven show/hide, icon click → viewer with stopPropagation, double-click fallback on/off. Adds a FakeResizeObserver and a synchronous requestAnimationFrame mock since jsdom doesn't ship either.

Reveal rules

Icon is shown when either:

• data type matches /text|blob|json|mediumtext|longtext/i and the formatted value is ≥ 20 chars and the value is non-null, OR
• ResizeObserver reports scrollWidth > clientWidth (actual visible overflow).

The icon disappears the moment the column widens enough to fit the value — no more stale heuristic.

Verification

Gate	Result
npx vitest run src/components/grid/__tests__/TruncatedCell.test.tsx	19/19 pass
npx vitest run (full suite)	77 files / 1453 tests pass
npm run type-check	0 errors
npx eslint (touched files)	0 errors (1 pre-existing warning in ResultsGrid unrelated)
npx dprint check (whole repo)	clean

Implementation notes

• ResizeObserver uses a flag-based rAF debounce rather than handle tracking. With a synchronous test mock, handle-tracking broke dedup because the mock returned the same id each call.
• Icon button stops propagation so single clicks on the icon don't trip the grid's row selection handler.
• Double-click on the text element is still wired to the viewer when the icon is shown — preserves muscle memory for existing users.
• truncate alone gives text-overflow: ellipsis overflow: hidden white-space: nowrap. The wrapping flex row uses min-w-0 so the inner truncate actually measures against the available column width.