Fix #139: SVG import paints paint-order="stroke" text strokes under the letters

[bflatastic]

Fixes #139: text with paint-order="stroke" imports with its stroke under the letters.

• The text import never read paint-order. When every character shares one stroke that paints before the fill, that stroke now becomes the text object's own stroke, placed below the Characters row, and the characters' own strokes are cleared. The canvas, SVG export and PDF export already draw this correctly.
• Gradient and pattern strokes are resolved in document space.
• If the characters' strokes differ, they are kept as they are and the import warns.
• Tests: crates/svg/tests/text_paint_order.rs, including a pixel probe that finds 411 yellow letter pixels with the fix and 0 without it.

Checks

• One of three stacked PRs (Bug: SVG import ignores vector-effect="non-scaling-stroke" (strokes are scaled by the transform) #131, Bug: SVG import ignores paint-order="stroke" on text (works on paths) #139, Bug: SVG and CSS export write Semibold text as bold, and Light or Medium as regular #136), on main at 088ffc8 (Fix #87: rust-version 1.95 and clippy 1.95 #144).
• cargo xtask ci passes all 7 steps (fmt, clippy, tests, assets, brands, layers, wasm) at the top of the stack, with 2,935 tests.
• Nothing dropped: no engine, UI or MCP code changed. Only new test files were added; no existing test changed.

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