SVG import: tspan line breaks follow the baseline gap (none on the same baseline, bounded far gaps)

[storybored379]

The SVG text fallback turns the gap between an absolutely positioned <tspan y> and the previous line into
that many line breaks. That has two problems:

• At least one break was added even for a tspan on the current baseline. Styled or kerned runs
  written with x and y on every tspan, as Illustrator writes them
  (<tspan x="0" y="0">Hello </tspan><tspan x="55.6" y="0" fill="red">world</tspan>), came back as one line per run:
  "Hello \nworld".
• The count had no upper bound. A 150-byte SVG with <tspan y="1e15"> asks for about 7·10¹³ '\n' characters.
  Opening it (File ▸ Open, MCP open_file or vectorcraft-cli convert) eats all memory and aborts with
  "memory allocation of … bytes failed".

What changed

• crates/svg/src/import.rs: a tspan less than half a line from the current baseline adds no break. The gap is
  clamped to 10 000 line breaks, and a non-finite gap counts as one. Other gaps are imported exactly as before.

How it was found and tested

• Both were found with synthetic SVG edge cases through vectorcraft-cli convert. The y="1e15" file aborted after 2 s under a
  4 GB ulimit -v; without a limit it keeps allocating.
• New test import_tspan_far_below_is_bounded (crates/svg/tests/roundtrip.rs) uses a tspan 2 000 000 units down. That is
  harmless on main (138 888 line breaks) but shows the missing bound. It also checks that an ordinary
  two-line gap still gives one blank line. On current main without the change it fails with "138888 line
  breaks". It passes with the change.
• New test import_tspan_on_the_same_baseline_stays_on_the_line covers runs on one baseline plus a following
  line. On main it fails ("Hello \nworld\nnext") and passes with the change.
• Full gates: cargo fmt --all -- --check (no diff in touched files; current main already has one in xtask/src/version.rs), cargo clippy --workspace --all-targets -- -D warnings (Rust 1.99, with
  chunks_exact_to_as_chunks/question_mark allowed because they already fire on main), cargo test --workspace,
  cargo xtask layers, cargo xtask assets, cargo xtask wasm.
• Files: crates/svg/src/import.rs, crates/svg/tests/roundtrip.rs
• Clean-room: no third-party source consulted.

Found and fixed with Claude Code (AI-assisted).

🤖 Generated with Claude Code

[echelon]

Thanks @storybored379, merged! Fixing both the same-baseline runs (common in real-world SVGs) and the unbounded allocation from a huge y in one small change is excellent. I confirmed both new tests fail on the old code.

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