fix: Instance-Accent PWA Titlebar Wash (Mock Parity)

[sahil-noon]

Meta

Change ID	Type	Confidence	Plan	Review
y5c3	fix	4.9/5.0	5/5 tasks, 11/11 acceptance ✓	✓ 1 cycle

Impact	+/−	Net
raw	+327 / −18	+309
true	+36 / −14	+22
└ impl	+29 / −13	+16
└ tests	+7 / −1	+6

_{excludes fab/, docs/ · generated by fab-kit v2.16.5}

Pipeline: intake ✓ → apply ✓ → review ✓ → hydrate ✓ → ship → review-pr

Summary

PR #435 wrote the full contrast-guarded accent hex (stripeHex) into the theme-color meta, so an installed PWA's titlebar painted the entire chrome in a loud, fully saturated hue — and the deliberate 2px accent stripe below it became invisible, since it abutted a titlebar of the identical color. This restores the reviewed design mock: a subtle dark blend of the accent into the theme background for the titlebar, with the full-brightness stripe visible directly beneath it.

Changes

• deriveAccentHexes gains a third derived hex — the meta/titlebar hex
• The blended titlebar hex — NOT stripeHex — becomes the meta content and the echo hex
• Unchanged — the other surfaces and the resolution chain
• Tests to update