M4: SVG import units and text fidelity (M4.18, M4.19)

[bflatastic]

Part of M4 (Files). SVG import keeps physical units and text fidelity: text stays in its place in the stacking order with its group, clip, mask and paints, and <textPath>, per-character positions, weights, spacing and CSS selectors survive. This ports an earlier session's work onto current main and reconciles it with #6, #8, #11 and the M3 import additions.

• M4.18: SVG import units.
  • A pixel or unitless user unit is 1 pt; absolute units keep their physical size at 72 pt per inch (12pt is 12 pt, 1in is 72 pt). Unitless 100 stays 100.
  • SVG import: root sizes in mm/cm/in/pt open at their physical size #11's root handling is kept: a root size in mm or inches opens at its physical size (210mm → 595.28 pt), with CSS-px user units.
  • The document's units follow the unit of the root width (Pixels without one).
  • Font sizes (em/ex/%, CSS size keywords), text positions, letter spacing, stroke widths and dashes follow their units.
• M4.19: SVG text import fidelity.
  • Placement: each <text> is swapped for a placeholder before parsing, so it keeps its z-order, parent group, clip, mask, opacity, <use> instances and <a href> link.
  • Paints: gradient and pattern paints resolve in text space.
  • Type on a path: <textPath> honours startOffset, side and the text anchor (this replaces SVG import: <textPath> comes back as type on a path #6's simpler version). Vertical writing-mode becomes type on a vertical path.
  • Character positions: per-character x/y/dx/dy/rotate become line breaks, kerning, baseline shift and rotation. SVG import: tspan line breaks follow the baseline gap (none on the same baseline, bounded far gaps) #8's cap on line breaks is kept, and far-off values stay finite.
  • Character styles: baseline-shift, numeric weights mapped to style names, letter and word spacing, kerning, and the character-stroke options are kept.
  • CSS selectors: type, #id, several classes, attribute, descendant and child selectors, with !important and specificity.
  • Patterns: <pattern> imports as a pattern swatch.

Known limits (documented in svg/src/lib.rs):

• Lines after the first start at the first line's x.
• Text inside a clip path is ignored.
• Absolute positions inside type on a path are ignored.

Checks

Rebased onto 39ce7b0 (#30). cargo xtask ci passes all 7 steps (fmt, clippy, test, assets, brands, layers, wasm).

• New tests: svg/tests/import_units.rs and import_text.rs, covering the spec's cases plus links, strokes, siblings and finite positions. The import fuzz test gains the new attributes and a hostile-CSS property test.
• Nothing dropped:
  • every test name on main still exists (1,913 → 1,942);
  • command ids, UI_COMMANDS and MCP tools are unchanged.
• Two changed tests in svg/tests/roundtrip.rs:
  • import_clip_mask_and_warnings now checks a pattern swatch is imported instead of a "pattern not supported" warning.
  • import_tspan_on_the_same_baseline_stays_on_the_line checks the red run's exact position, since "world" now sits at its exact x.

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