fix(php-transformer): include table-cell box model in layout-table tracks

[chubes4]

Problem

After #2211, sized layout-table columns keep their authored CSS width (flex-basis + flex-grow:0). A fixed-layout post + sidebar row still came out 10px too wide in the leftover (unsized) column.

Measured at 1440×900 on a public two-column blog post:

	Source	WordPress after #2211
Post column (content-box)	735px	745px
Sidebar	215px at x=981	215px at x=985
First photo	735×490	745×496

The leftover track was container − 215, ignoring the table box model. Photos sized to the post column inherited the extra 10px.

Root cause

A CSS width on a td is content-box. Flex columns are border-box with box-sizing:border-box. Copying 215px as flex-basis made the sized track 2px too small (UA td padding is 1px per side) and dropped UA border-spacing:2px (engine-support forces gap:0 to beat Gutenberg's 2em).

On a 960px table-layout:fixed row:

• Source: 2px spacing + 737px post border-box + 2px spacing + 217px sidebar border-box + 2px spacing = 960. Post content-box 735, sidebar at x=981.
• After fix(php-transformer): preserve layout-table cell track widths #2211: 745 + 215 = 960. No spacing, no cell padding.

Stage: php-transformer layout-table → columns lowering (layoutTableCellTrackWidth / layoutTableColumnsAttributes / engine-support CSS). Not capture.

Fix

• Absolute cell tracks include horizontal padding (UA 1px per side when unset, or authored/cellpadding) and border widths, converting content-box width to the border-box flex-basis.
• Percent tracks stay as-is (padding remains inside, matching existing contract).
• :where() restores UA table-cell padding:1px at zero specificity so authored padding still wins.
• Separate-border leftover rows (an unsized column remains) reconstruct spacing as column-gap + padding-inline on a carrier whose selector beats the gap:0 reset. Collapsed and all-percent rows keep gap:0.
• Box 1–4 value shorthands expand through one helper, CssValueInspector::expandBoxShorthand() → [top, right, bottom, left]. Layout-table track sums and StyleAttributeMapper::boxSides() both use it. Border-width tokens in border shorthands go through CssValueInspector::isBorderWidthToken().

No product or platform names.

Test

php-transformer/tests/unit/layout-table-column-track-width.php

• Fails on trunk: content-box pixel cell width plus ua padding becomes the border-box column track (got 215px, want 217px) plus HTML width, authored padding, UA padding CSS, and leftover spacing (6 failing assertions).
• Passes with the fix: 14 assertions (CSS px, HTML width, authored padding:0 keeps 215px, authored padding:0 4px becomes 223px, leftover spacing, collapsed/percent rows do not emit UA gap).

php-transformer/tests/unit/css-value-inspector.php covers the shared side-expansion helper.

composer test, php tests/contract/production-acceptance-matrix.php, and visual-parity npm test passed locally.

Before / after (Playwright, 1440×900 unless noted)

Post: /blog/experiencing-the-rural-life-of-100-years-ago/

	Post content-box	Sidebar x	First photo	scrollWidth @390
Source	735	981	735×490	—
After #2211	745	985	745×496	390
This PR	735	981	735×490	390 (no overflow)

/blog/ at 1440: post 735, sidebar x=981. 768 and 390 stack (flex-basis:100%), no overflow.

Block editor (rural-life post): 97 blocks, all isValid.

AI disclosure

Written by Claude (Anthropic) via Claude Code, orchestrated by chubes4. Claude measured source vs WordPress table geometry (content-box width, UA padding, border-spacing), extended layout-table column lowering to include the cell box model and leftover-row spacing, added a regression test that failed on trunk, ran php-transformer suites, built an SSI dev package from this worktree, imported the kept capture, and measured before/after including Gutenberg isValid. Review follow-up extracted CssValueInspector::expandBoxShorthand() / isBorderWidthToken() so track sums and StyleAttributeMapper share one side-expansion implementation.

[chubes4]

Reviewed and reproduced: the root cause is right (a td's CSS width is content-box; the UA 1px cell padding and 2px border-spacing were dropped, leaving the unsized track 10px wide), and the fresh-import numbers match the source exactly.

One change before merge: this adds a new padding/border shorthand parser (layoutTableCellInlinePadding, layoutTableCellInlineBorderWidths) while StyleAttributeMapper::parseBorderShorthand() already exists and several call sites already enumerate padding sides. Please reuse or extract a single shared side-expansion helper (e.g. on CssValueInspector) and build the track sum on it, so there is one shorthand implementation. Behavior and tests stay the same.

Review by Claude (Anthropic) via Claude Code, orchestrated by @chubes4: AI read the diff, checked for existing primitives, and drafted these notes.