fix(pptx): preserve pair kerning across spaces

[developer0hye]

Summary

Preserve the active font's pair kerning on both sides of PowerPoint spaces while retaining the 1/8pt advance grid. Arial on Aug now fits an 88.5pt box as native PowerPoint does; previously it wrapped before Aug. Combine pair and grid adjustments in one weak spacing element so both disappear at line boundaries and spaces remain searchable.

Related: #1582.

Testing

• Two portable regressions failed before the fix and pass after it: both sides of spaces, repeated spaces, kerning disabled, two font sizes, wrapping and new-line origins.
• cargo test --locked --workspace --no-fail-fast: all integration/doc suites passed; one old marker helper-call assertion required updating. The corrected cargo test --locked --workspace --lib passed all 2,892 library tests, including observable marker/body placement. Combined validation: 3,491 passed, 15 ignored.
• cargo clippy --locked --workspace --all-targets -- -D warnings, cargo fmt --all -- --check, and git diff --check passed.
• Twenty-nine native PowerPoint 16.112.3 probe exports match candidate line contents. Checked word origins differ by at most 0.011pt horizontally and 0.12pt vertically. All native integrity and re-zip control gates passed.
• Sixteen-page layout audit passes at 0.5pt; all strict pixel-cluster reports pass. Searchable text matches all 2,337 normalized codepoints.
• Documentation freshness review: PASS. Reproduction and provenance are in the issue evidence README.

Visual impact

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

Visual audit

• Issue: PPTX: word-level width rounding drops kerning across spaces #1582
• Fixture: tests/golden_mocks/business/sources/pptx/03_product_launch_en.pptx; one-factor letter and box-width probes reproduced in the evidence README
• Page(s): 1-16
• Renderer and DPI: pdftoppm, 150 DPI; native PowerPoint 16.112.3
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1582/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1582/render-clusters-page-1.json, assets/bugfixes/issue-1582/render-clusters-page-2.json, assets/bugfixes/issue-1582/render-clusters-page-3.json, assets/bugfixes/issue-1582/render-clusters-page-4.json, assets/bugfixes/issue-1582/render-clusters-page-5.json, assets/bugfixes/issue-1582/render-clusters-page-6.json, assets/bugfixes/issue-1582/render-clusters-page-7.json, assets/bugfixes/issue-1582/render-clusters-page-8.json, assets/bugfixes/issue-1582/render-clusters-page-9.json, assets/bugfixes/issue-1582/render-clusters-page-10.json, assets/bugfixes/issue-1582/render-clusters-page-11.json, assets/bugfixes/issue-1582/render-clusters-page-12.json, assets/bugfixes/issue-1582/render-clusters-page-13.json, assets/bugfixes/issue-1582/render-clusters-page-14.json, assets/bugfixes/issue-1582/render-clusters-page-15.json, assets/bugfixes/issue-1582/render-clusters-page-16.json
• Reference exporter differences: None
• Fine-detail threshold: 0.5pt
• 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 context, full-scale body crops and 5% pixel sweeps show corrected suffix spacing and two-line wraps. Heading and timeline regions, bullets, indentation, font emphasis and chart hairlines remain intact. One unchanged gold legend-edge cluster on page 3 is explicitly accepted as photo-resampling; bounded body glyph-edge fragments remain below the material floor. No remaining converter deviation was found.
• GT: assets/bugfixes/issue-1582/gt.jpg
• Before: assets/bugfixes/issue-1582/before.jpg
• After: assets/bugfixes/issue-1582/after.jpg
• Native: None
• Compare: assets/bugfixes/issue-1582/compare.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: all sixteen collection pages
Element presence	Matches GT: headings, timeline, chart and bullets
Position/size	Fixed: space-pair origin drift and resulting wraps
Rotation/flip	No deviation observed
Fill	Matches GT: unchanged fills; accepted legend-edge resampling
Stroke/border	Matches GT: fine axes, ticks and legend outlines
Shape outline geometry	Matches GT
Text content	Matches GT: exact normalized searchable text
Font family/weight/style	Matches GT: Arial body and heading emphasis
Text color	Matches GT
Alignment	Matches GT: preserved bullets and new-line origins
Line/paragraph spacing	Matches GT: native baseline quantization within 0.12pt
Clipping/overflow	No deviation observed

Checklist

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