fix(pptx): round hard-break seats per line

[developer0hye]

Summary

• evaluate PowerPoint's rounded hard-break seat at each physical line's own maximum font size
• preserve the existing paragraph-relative model when explicit line spacing is present
• add exact mixed-size regression coverage plus a minimal native PowerPoint fixture and strict visual evidence

Related issue

Related: #1252

Testing

• cargo test --locked --workspace
• CARGO_TARGET_DIR=/private/tmp/office2pdf-1252-check cargo check --locked --workspace
• CARGO_TARGET_DIR=/private/tmp/office2pdf-1252-check cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• focused hard-break unit tests (6 passed), fixture smoke test, and native PowerPoint PDF comparison
• python3 scripts/compare_layout.py /private/tmp/office2pdf-1252-visual.aQu2pO/gt-isolated.pdf /private/tmp/office2pdf-1252-visual.aQu2pO/after-isolated.pdf --json --audit --fine-shift 0.12
• python3 scripts/compare_render.py /private/tmp/office2pdf-1252-visual.aQu2pO/gt-isolated.pdf /private/tmp/office2pdf-1252-visual.aQu2pO/after-isolated.pdf --page 1 --dpi 300 --artifacts-dir /private/tmp/office2pdf-1252-visual.aQu2pO/audit-isolated-page-1 --cluster-report assets/bugfixes/issue-1252/render-clusters-page-1.json --strict-clusters
• magick compare -metric AE -fuzz 5% (before: 14,024 pixels; after: 12,754 pixels versus GT)

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: Mixed-size hard-broken PPTX paragraphs now use PowerPoint's per-line rounded seat.

Visual audit

• Issue: PPTX: a hard break between two sizes seats the following line on the paragraph's split, not on its own size #1252
• Fixture: tests/fixtures/pptx/hard_break_mixed_size_seat.pptx
• Page(s): 1
• Renderer and DPI: native Microsoft PowerPoint GT export; Poppler pdftoppm, 300 DPI
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1252/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1252/render-clusters-page-1.json
• Reference exporter differences: None
• Fine-detail threshold: 0.12pt
• Text-layer-only: No
• Pixel delta: 5,056 pixels between before and after at 5% fuzz
• Layout audit page count: Pass
• Layout audit text flow: Pass
• Layout audit visible fills: Pass
• Layout audit rectangle geometry: Pass
• Layout audit large shifts: Pass
• Layout audit fine shifts: Pass
• New follow-up issues found in this audit: None
• Model vision findings: Full-page, diff, and full-resolution crop inspection shows the bold Arial caption and all nine alternating 20pt/10pt lines present and aligned. The prior small-line baseline offset is gone; the after page matches GT within 0.08pt, with only dispersed glyph-edge rasterization in the diff and no material clusters. No clipping, border, fill, rotation, or emphasis mismatch remains.
• GT: assets/bugfixes/issue-1252/gt.jpg
• Before: assets/bugfixes/issue-1252/before.jpg
• After: assets/bugfixes/issue-1252/after.jpg

Visual comparison

GT	Before	After

Required inspection

• Rendered all evidence at 150 DPI or higher
• Stored progressive JPEG quality 86 assets with metadata stripped
• Used Codex/Claude vision to inspect the full GT/output pages, diff, and matched crops
• Inspected matched region crops at full resolution
• Ran compare_layout.py --audit --fine-shift PT and dispositioned every fine/large text-instance shift, rectangle geometry deviation, painted-text visibility mismatch, and visible-fill occlusion
• Ran compare_render.py --cluster-report PATH --strict-clusters and dispositioned every material 5% fuzz diff cluster by explicit ID
• Inventoried hairlines and border dash styles
• Inventoried font weight, italic, and underline emphasis

Deviation audit

Check	Result
Page count/order	Matches GT: one page in the same order.
Element presence	Matches GT: the caption and all nine body lines are present.
Position/size	Fixed: small-line baseline errors up to 0.46pt are removed; worst after baseline delta is 0.08pt and worst text-width delta is 0.163%.
Rotation/flip	No deviation observed: the fixture has no rotated or flipped content.
Fill	Matches GT: black text on the white slide background.
Stroke/border	No deviation observed: the fixture contains no visible strokes, borders, dash styles, or hairlines.
Shape outline geometry	No deviation observed: the text boxes have no visible outline, and the matched page rectangle agrees within 0.00001pt.
Text content	Matches GT: normalized text and all ten visible lines are identical.
Font family/weight/style	Matches GT: Arial is preserved; the caption is bold and all body runs are regular, with no italic or underline runs.
Text color	Matches GT: all visible text is black.
Alignment	Matches GT: caption and body lines remain left aligned.
Line/paragraph spacing	Fixed: each 20pt/10pt hard-break line now uses its own whole-point seat; all ten baselines match GT within 0.08pt.
Clipping/overflow	No deviation observed: every glyph is fully visible inside the slide.

Checklist

• Commits include a Signed-off-by line
• PR scope contains one root cause
• Remaining visual deviations each reference an open issue

[developer0hye]

Exact-head CI run 33821140488 is blocked by the known upstream tinyvec 1.13.0 regression: dependency compilation fails at tinyvec.rs:710 because the vec! macro is not in scope. The failure occurs before this PR's Rust checks/tests can run.

The commit-local gates that do not compile that dependency are green: DCO, Format, Release Automation Contract, Business Golden Contract, and Visual PR Contract. Local cargo test --locked --workspace, isolated check/clippy, and the strict visual audit passed against 6f07d5cbe12e3c3a4fc15a754c47298e0f5cea96 using the pinned pre-regression graph.

Upstream fix: Lokathor/tinyvec#226. This PR will remain unmerged until the consumer graph is healthy and exact-head CI is rerun.