Render whitespace glyphs in unified-diff code minings and overlay

[tobiasmelcher]

Fixes #2942

When "Show Whitespace Characters" is enabled, the unified-diff deletion and addition code minings now render whitespace as visible glyphs at the configured alpha, matching the editor text drawn by WhitespaceCharacterPainter:

• spaces as ·, the ideographic space (U+3000) as °
• tabs as a single » on the first column of the expanded tab run
• ¤/¶ line delimiters at the end of filled lines

The leading/enclosed/trailing (and ideographic) visibility flags are honored as the editor does, including on all-whitespace lines where any of the three flags shows the run.

Because the label is kept tab-expanded for layout, the glyph substitution learns from the raw line which expanded columns originate from a tab. The click-through overlay opened on a mining keeps the original tabs in its document and lets the real WhitespaceCharacterPainter render them.

The global whitespace settings are read once and cached, invalidated by a preference-store listener, instead of being re-read on every paint of every mining.

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Developed with assistance from Claude Code.

[github-actions]

Test Results

    51 files  ± 0      51 suites  ±0   58m 40s ⏱️ + 1m 19s
 4 493 tests +17   4 471 ✅ +17   22 💤 ±0  0 ❌ ±0 
11 568 runs  +51  11 414 ✅ +51  154 💤 ±0  0 ❌ ±0 

Results for commit 34df500. ± Comparison against base commit 8b229d0.

♻️ This comment has been updated with latest results.

[tobiasmelcher]

@HannesWell it would be great if you could review and test this change by end of next week. I'll be on vacation then and plan to merge once I'm back.