Keep class-owned max-width off core/column serialized style

[chubes4]

Problem

Fixture 37 now emits an editor-invalid core/column because class-owned max-width: var(--essay) is serialized onto the column wrapper in addition to its generated geometry carrier. Gutenberg core/column save output does not include that unsupported inline max-width, so validation fails.

Evidence

SSI eight-surface proof: Homeboy run 4610bb1b-6a98-40fb-b2b6-bed00bc1fba3.

Observed mismatch:

core/column
expected style: margin-top:0;...;padding-left:0
received style: margin-top:0;...;padding-left:0;max-width:var(--essay)

Source CSS:

.essay__body {
  grid-column: 2;
  max-width: var(--essay);
}

BlockFactory already excludes max-width from serialized styles for core/group, core/columns, core/list-item, core/paragraph, and core/separator, retaining it through generated geometry CSS. core/column is omitted from that policy even though it uses the same carrier path.

Acceptance criteria

• Class-owned max-width on a source element converted to core/column is retained through generated geometry CSS, not unsupported serialized block style.
• The transformed wrapper remains visually equivalent.
• Gutenberg validation reports the fixture-37 column valid.
• Add focused canonical save-shape and geometry parity coverage.
• Existing PHP transformer contract and canonical suites pass.

Related

• Keep separator geometry block-valid #704 fixed the same ownership class for core/separator.
• Declare forms capability for fixture 37 and verify Jetpack materialization #700 tracks the fixture-37 form lifecycle proof.

AI assistance

• Model: OpenAI GPT-5.6 Sol
• Tool: OpenCode and Homeboy
• Used for: ran the eight-surface SSI proof, isolated the exact save-shape mismatch, and traced the missing block-policy entry. Chris Huber owns review and decisions.