test: HWPX SVG snapshot regression harness (closes #173)

[seunghan91]

Summary

• Adds tests/svg_snapshot.rs — a pure-Rust integration test that renders HWPX pages via HwpDocument::render_page_svg_native and diffs the output against committed golden SVGs.
• Ships the first golden (form-002.hwpx page 0) to establish the baseline.
• Runs under the existing cargo test step in ci.yml, so the check is active on every PR with no new workflow and no Hancom-Office / Windows dependency.

Closes #173.

Design

Pattern borrowed from the MDM project's core/tests/golden_hwpx.rs (same maintainer). Two things worth calling out:

• UPDATE_GOLDEN=1 regeneration. When a rendering change is intentional, run UPDATE_GOLDEN=1 cargo test --test svg_snapshot to rewrite the goldens in place. The diff gets reviewed in the same PR as the source change.
• Determinism probe. render_is_deterministic_within_process renders the same page twice inside a single process and asserts byte equality. If this ever fails, every snapshot test above becomes unreliable regardless of golden correctness — so we surface non-determinism as its own failure mode.

Font embedding is explicitly OFF (the native entry point doesn't embed), which keeps goldens independent of host system fonts.

Verification

• Byte-identical across two back-to-back CLI invocations on the same machine:

  rhwp export-svg samples/hwpx/form-002.hwpx -o /tmp/a -p 0
  rhwp export-svg samples/hwpx/form-002.hwpx -o /tmp/b -p 0
  diff -q /tmp/a /tmp/b   # empty

• cargo test --release --test svg_snapshot passes locally (macOS, 2 tests).

Test plan

• CI green on cargo test
• Review the committed golden (tests/golden_svg/form-002/page-0.svg) visually — it's ~263 KB of readable SVG.
• If golden size concerns arise later, gzip-encoding is a one-line change to the harness.

Not in this PR

• Expanding the golden corpus to the 2024/2025 investment samples — easier to do file-by-file in follow-up PRs once the first run is green.
• Cross-platform determinism check (macOS vs ubuntu CI) — will surface itself the first time CI runs the harness.

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