Fix paragraph save-shape regressions in SSI fixtures

[chubes4]

Problem

The focused SSI fixture matrix still emits invalid core/paragraph blocks on current Blocks Engine origin/trunk (c6bbd82a), despite #577 and #593 being present.

Affected fixtures:

• 10-nonprofit: class="has-text-color has-text-color hero-sub"
• 13-realistic-small-business: class="has-text-color has-text-color lead hero-desc"
• 13-realistic-small-business: generated be-inline-geometry-* class and max-width:380px do not match the block comment/save shape
• 74-lumen-coffee: class="has-text-color has-text-color hero__lead"

The real editor validation reports four invalid blocks:

Fixture	Valid	Invalid
10-nonprofit	210	1
13-realistic-small-business	217	2
74-lumen-coffee	117	1

All three otherwise retain 100% native conversion and zero core/html fallback in the measured front-page documents. The failure is deterministic serializer/save-shape disagreement, not visual parity gating.

Expected

Paragraph color and geometry conversion must produce block comment attributes and rendered HTML that round-trip through wp.blocks.validateBlock() without duplicate support classes or implementation-only geometry attributes.

Acceptance criteria

• The three named fixtures produce zero invalid editor blocks.
• Preset slug text produces exactly one has-text-color class while preserving author classes.
• Paragraph margin-top:1rem;max-width:380px preserves geometry through a save-shape-compatible carrier without adding attributes core's save function omits.
• Existing Fix PHP transformer text color class validity #577/Preserve inline geometry in generated block assets #593 unit coverage remains green and gains an end-to-end serialized-block validation regression covering the actual fixture path.
• php-transformer tests, fixture matrix tests, and git diff --check pass.

This is a follow-up regression to #577 and #593 discovered by the four-fixture attribution wave.