fix(docx): autofit table columns by compressible slack, not a uniform scale

[developer0hye]

Summary

A w:tbl without w:tblLayout fixed is auto-fitted by both Word and office2pdf — by different rules. We scaled per-column tcW maxima uniformly: the invoice's conflicting 4200-twip claims put Description and Amount at an identical 161.32pt (the equal-share signature), Unit Price lost 9.6pt, a wrap flipped, and the lower half of the page sat 11.3pt high.

Word compresses each grid column in proportion to its compressible slack above min-content, never below it: k = (W − Σmin)/(Σpref − Σmin), width_i = min_i + (pref_i − min_i)·k. Preferences track grid-column occupancy through gridSpan; min-content is the widest unbreakable token measured with the run's resolved face/weight/size plus tcMar (every CJK character is its own token; NBSP does not break). W is tblW capped at the grid sum. Derived and verified on the invoice's Word GT: every column within 0.10pt, the row-2 wrap flips to Word's two lines, the totals ride the col-4/5 boundary within 0.28pt.

Measurement uses a new cached render::pdf::text_advance_em (hmtx sums, per-(family, weight) face cache, wasm None twin, embedded/user font paths documented as the same accepted limitation as #621's accessor). The model runs only in the GT-verified compression direction; conflict-free tables return the grid verbatim; any unmeasurable token degrades to the exact pre-change uniform scale — all nine other business docx mocks and the 39-page technical brief are byte-identical to main.

Related issue

Fixes #624

Testing

• cargo test --workspace — green (11 red-first unit tests: slack distribution, gridSpan occupancy, min floors, grid-verbatim, degrade paths, CJK singleton tokens — the reviewer's 200pt-table CJK probe no longer overflows to 377pt — NBSP tokens, tblW cap, undershoot gate, all-empty parity; text_advance_em pins incl. bold-face selection)
• cargo +1.97 clippy --workspace --all-targets — clean; cargo fmt --check — clean; wasm check clean
• GT: invoice columns 27.93/156.99/47.74/65.24/153.40 vs Word 27.9/156.9/47.8/65.3/153.3; table bottom 334.26 vs 334.56 (was 11.2pt high); compare_layout 14/14 lines, 0 re-wrapped

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason:

Visual audit

• Issue: DOCX: an auto-layout table splits its columns evenly, flipping a wrap and lifting the rest of the page 11.3pt #624
• Fixture: tests/golden_mocks/business/sources/docx/01_invoice_en.docx
• Page(s): 1
• Renderer and DPI: pdftoppm, 150 DPI
• Evidence mode: fix
• New follow-up issues found in this audit: DOCX: table borders are centred on the cell boundary instead of edge-aligned, so the grid sits 0.1-0.2pt left of Word #724, DOCX: a footer '- PAGE -' paragraph sets 17% narrower than Word, tightening the spaces around the field #725
• GT: assets/bugfixes/issue-624/gt.jpg
• Before: assets/bugfixes/issue-624/before.jpg
• After: assets/bugfixes/issue-624/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
• Inspected matched region crops at full resolution
• Ran the 5% fuzz pixel-difference sweep
• Inventoried hairlines and border dash styles
• Inventoried font weight, italic, and underline emphasis

Deviation audit

Check	Result
Page count/order	Matches GT — one A4 page each
Element presence	Matches GT — all elements and all 14 grid hairline segments present (GT's higher rect census is per-segment op-splitting, confirmed visually)
Position/size	Remaining: #649 — cell text 0.37–0.50pt high / 0.14–0.35pt left of Word's inset; Remaining: #638 — the gap under 'Bill To' is 0.41pt short. The column widths this PR fixes are within 0.09pt of Word and the table bottom within 0.30pt (was 11.2pt high)
Rotation/flip	No deviation observed
Fill	Remaining: #647 — band colours match; shading spans centreline-to-centreline where Word fills to the border's inner edge (0.24pt/side)
Stroke/border	Remaining: #724 — borders centred on the boundary instead of edge-aligned, grid 0.1–0.2pt left of Word; all segments present, weights within 0.02pt
Text content	Fixed — the row-2 wrap now matches Word ('Additional render workers (4' / 'vCPU)'); every other string identical
Font family/weight/style	Remaining: #628 — family/weight/style match, but unconditional pair kerning sets runs 1.0–1.4% tighter
Text color	Matches GT
Alignment	Remaining: #646 — the centred footer field sits 0.74pt right; Remaining: #725 — the '- PAGE -' footer paragraph sets 17% narrower. The totals this PR fixes align within 0.28pt (was 8.1pt left)
Line/paragraph spacing	Remaining: #638 — wrapped-cell pitch 0.12pt tight and inter-block gaps 0.28–0.41pt short (max-ascent/descent line-box model)
Clipping/overflow	No deviation observed

Checklist

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

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