fix(php-transformer): carry social icon color into shared header parts

[chubes4]

Problem

After #2234, uniformly coloured social icons in page content render with the authored colour, but a shared header extracted to a template part still emits core/social-links without iconColorValue. Pages that use that part keep core's per-service brand colours.

Root cause

The colour carry reads a monochrome glyph from asset metadata keyed for the compiling document. A root-relative icon URL matches that map on the entry document, whose directory is the web root, and misses it on every nested document. Shared-shell extraction then takes the dominant nested-page header, so the template part never receives the colour. Isolated shared-shell compilation also never received the payload reader the page path uses to sample referenced glyphs. Placeholder icons in the extracted header (a 1×1 stand-in for the same destination) had no local pixels to sample even when the real glyph was elsewhere in the document.

Fix

Extend the existing glyph-colour primitive, without a second mechanism:

• Index each image under its web-root URL so every document resolves the same root-relative glyph.
• Give isolated shared-shell compilation the same payload reader the page path already uses.
• When a local icon image has no authored paint, reuse the monochrome colour of another image for that same destination.

Regression test

php tests/unit/social-links-icon-color.php

Fails before the fix:

FAIL: a shared header template part carries the root-relative glyph color, got <!-- wp:social-links {"className":"social-links is-style-logos-only blocks-engine-source-social-item-spacing","size":"small"} -->
EXIT:1

Passes after:

Social-links icon color tests passed

The added cases cover an embedded root-relative glyph, placeholder icons that share a destination with a real glyph, and a payload-referenced staged import. Mixed icon colours still leave core service colours alone.

Verification

Fresh import of the private capture with Static Site Importer origin/main @ 6f0eff4 and this branch @ 6424fc2. The dev-package ZIP vendors the change (sameDestinationGlyphColor, web-root lookup key, shared-shell payload reader).

Inner page that renders the header template part, 1440px, three header icons, 20×24 at x 1248 / 1278 / 1308, y 213 on both sites:

	baseline	this branch
first	rgb(255, 0, 0)	rgb(255, 255, 255), has-icon-color
second	rgb(240, 0, 117)	rgb(255, 255, 255), has-icon-color
third	rgb(8, 102, 255)	rgb(255, 255, 255), has-icon-color

Source header icons at the same x positions are 20×20 white glyphs. At 390 both sites collapse the header icons to 0×0; computed colour stays the brand colours on the baseline and rgb(255, 255, 255) here. parts/header.html now has one wp:social-links with iconColorValue #ffffff.

Also passed: php tests/unit/social-links-boundary.php, php tests/contract/shared-shell-plan.php.

Fixes #2238

AI disclosure: implemented by xAI Grok 4.7 via OpenCode (opencode run), orchestrated and reviewed by Claude (Anthropic).