test(grid): coverage for virtualization, compression math, viewport priority

[merlin1de]

Summary

Closes the test gaps identified after the mobile-scroll session (#73). No behavior change in production code — pure refactor + new tests.

Refactor (no-op, single source of truth)

New src/utils/gridLayout.js with pure helpers extracted from `GridView.vue`:

• `spacerHeightForRows(n, rowHeight, gap)`
• `computeCompressionRatio(fullLogicalHeight, maxPhysicalHeight)`
• `computeTopSpacerHeight({...})` — both classic and continuous (compression) branches
• `iterateViewportFirst(rangeStart, rangeEnd, vpStart, vpEnd)` generator yielding `[index, priority]` pairs

`GridView.vue` delegates to these. Existing 418 Vitest tests still green.

Vitest +24

`tests/js/gridLayout.spec.js` (20 tests)

• `spacerHeightForRows` edge cases (n ≤ 0, N×rowHeight + (N-1)×gap match)
• `computeCompressionRatio` cap behavior + broken-cap fallback
• `computeTopSpacerHeight`:
  • Fallback (virtualEnabled = false) returns 0
  • Compression = 1 (classic): matches `spacerHeightForRows`, independent of scrollTop
  • Compression > 1: top-edge clamp at `visibleStartRow ≤ 0`, sub-row offset, negative-formula clamp
  • Continuity check: at Δ scrollTop = 1 within a row, Δ spacer ≈ `1 - compressionRatio` — proves the smooth scroll math
• `iterateViewportFirst` order, edge cases (no buffer above/below, empty range), and unshift-semantics simulation showing top-of-viewport ends up at queue front

`tests/js/GridView.spec.js` (4 tests)

• ↑/↓ keyboard navigation steps by exactly `columnsCount` items (regression guard for the `columnsEstimate` bug — was using `offsetWidth/THUMB_SIZE` returning 1 in jsdom and on mobile, which broke ↑/↓ on 2-column mobile layout)
• Bottom-edge clamp

Cypress +3

`tests/e2e/mobile-virtualization.cy.js` at viewport 414×896

• Virtualization activates when folder exceeds the visible range (rendered count < total, spacer present)
• Scrolling shifts the render range (data-index of first item moves forward)
• `topSpacer` appears after scrolling out of the top region

Skips with log message if test folder has < 20 images. Column count is not asserted — Cypress' viewport setting controls `window.innerWidth` but NC's mobile UI behavior in iframe varies; the virtualization mechanism is what matters.

Test plan

• Vitest: 442 / 442 (was 418, + 20 gridLayout + 4 GridView)
• Cypress: `mobile-virtualization` 3 / 3 against sixpack
• Cypress full suite — would expect same as PR fix(grid): smooth mobile scroll in huge grids #73 (only the known guest-gallery timing flake)