refactor(render): Modularize SVG and HTML renderers and improve CJK label width estimation

[mhiro2]

Summary

• Replace the ASCII/non-ASCII heuristic in SVG label width estimation with unicode-width for
  correct CJK and emoji rendering
• Add regression tests verifying HTML escaping of special characters in title and heading
  elements
• Split relune-render-svg lib.rs into node.rs and markers.rs to reduce file size and
  improve maintainability
• Split relune-render-html html.rs into css.rs and components.rs to separate concerns

Changes

• 3fb4475 fix(render-svg): use unicode-width for label estimation and avoid format!() in column
  loop
  • Replace the binary ASCII/non-ASCII heuristic in estimate_label_width with unicode-width's
    width_cjk() for correct widths of CJK, emoji, and other multi-cell characters
  • Eliminate per-column String allocation in the column rendering loop by writing directly
    into the output buffer with write!() instead of building an intermediate String via
    format!()
• b6c60c3 test(render-html): add regression test for title/heading escape of special characters
  • Verify that user-supplied titles containing HTML injection payloads, ampersands, quotes, and
    apostrophes are properly escaped in both the <title> and <h1> elements
• a647777 refactor(render-svg): split lib.rs into node.rs and markers.rs
  • Extract node rendering, column badges, label estimation, and overlay badge helpers into
    node.rs
  • Extract Crow's Foot inline marker rendering and sampling helpers into markers.rs
  • lib.rs shrinks from ~2500 to ~1900 lines, retaining the public API, defs, edge rendering,
    and tests
• 18f24fa refactor(render-html): split html.rs into css.rs and components.rs
  • Extract the ~1460-line build_css function into css.rs and the HTML/JS component builders
    into components.rs
  • html.rs shrinks from ~2400 to ~700 lines, retaining the document assembly logic and tests

[github-actions]

Code Metrics Report

	main (c1721cb)	#47 (3baaae4)	+/-
Coverage	94.1%	94.1%	-0.1%
Test Execution Time	1m35s	1m39s	+4s

Details

  |                     | main (c1721cb) | #47 (3baaae4) |  +/-  |
  |---------------------|----------------|---------------|-------|
- | Coverage            |          94.1% |         94.1% | -0.1% |
  |   Files             |             73 |            77 |    +4 |
  |   Lines             |          29862 |         29895 |   +33 |
+ |   Covered           |          28117 |         28148 |   +31 |
- | Test Execution Time |          1m35s |         1m39s |   +4s |

Code coverage of files in pull request scope (98.2% → 98.2%)

Files	Coverage	+/-	Status
crates/relune-render-html/src/components.rs	100.0%	+100.0%	added
crates/relune-render-html/src/css.rs	100.0%	+100.0%	added
crates/relune-render-html/src/html.rs	99.7%	-0.2%	modified
crates/relune-render-html/src/lib.rs	100.0%	0.0%	modified
crates/relune-render-svg/src/lib.rs	97.9%	+1.7%	modified
crates/relune-render-svg/src/markers.rs	84.5%	+84.5%	added
crates/relune-render-svg/src/node.rs	91.4%	+91.4%	added

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Reported by octocov