Trim file paths in the discard dialog, and cut trimmed paths flush by measurement

[danipen]

What

Two path-display fixes, one visible in the discard confirmation and one across every file list:

1. Discard dialog file names were invisible. The confirmation list showed full paths clipped from the right, so deep trees rendered every row as Modules/AssetBundle/Tests/UTFTests/AssetBundles/R…. Rows now use the same dir/name trimming as the pending-changes list: the directory gives way, the file name always stays visible.

2. The ragged gap between the trimmed directory and the file name is gone. CSS text-overflow: ellipsis drops the whole partially-clipped character and draws … where the last full one ended, leaving a row-varying blank between the dimmed directory and the name. The new TrimmedPath primitive measures the text instead (canvas measureText, binary search in pathTrim.ts) and cuts the directory string itself, so the name sits flush after the ellipsis in every row.

How

• pathTrim.ts — pure cut logic (measurer injected, bisects over code points so surrogate pairs never split), unit-tested including a linear-search cross-check.
• TrimmedPath.tsx — dim dir + strong name spans; fits before paint on mount/path change and re-fits via ResizeObserver; drops the tooltip layer's data-tip-overflow gate while trimmed (nothing CSS-clips once the text is cut to size, so the gate would suppress the full-path tip).
• Adopted by WorkingFileList rows (Changes, History, Graph and stash lists) and the discard dialog list; .tpath lives in primitives.css per the reuse rule.

Diff/conflict headers keep plain CSS ellipsis: their containers' widths are content-dependent, so measured trimming there needs its own layout rework (and the artifact is a single line, at most one character wide).

Verification

• lint, typecheck, test green (832 tests).
• Driven on screen via Playwright against a repo with 10 deep Unity-style paths: changes list and discard dialog render flush seams at multiple window sizes, zero console errors.

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