style(banner): match reference artwork with layered additive gold caption

[leochiu-a]

Summary

Rework the canvas-rendered approved/victory banner caption so it reproduces the image-creator's Elden "Victory" look on GitHub's dark theme, matching the reference banner artwork in src/assets/*.png.

The caption is two layers:

• Face — the image-creator's deep matte gold 220,175,45, a single clean opaque pass.
• Sheen — the warm ghost 255,208,66 @ 0.18, stacked on top of the face and blended additively via mix-blend-mode: plus-lighter (falling back to screen).

Where the sheen aligns over the face it adds to it — the bright overlap tone (~255,213,57); where it spreads past the letters it becomes the dim offset echo. So overlapping text shows a real blended colour (the layered-gold look) instead of flat gold. The sheen stays a separate layer (stacking order band → caption → sheen) so its outward spread (scaleX → 1.11) can animate after the fade-in.

Why

The old caption read wrong on GitHub's dark theme (variously too dark, too bright, or flat). Root causes found along the way:

• The reference PNGs have transparent backgrounds, so viewing the file over white looks like "grey band + matte gold," but overlaid on the dark page the true target is a deep matte gold — verified by compositing the reference PNG and the canvas render on the same dark background.
• The signature look comes from the additive ghost: overlap → brighter, offset → dim echo. screen can't reproduce that additive brightening; plus-lighter (true additive) can, and renders correctly in Chrome.

Changes

• eldenBanner.ts — face = single opaque deep gold; sheen = additive ghost 255,208,66 @ 0.18.
• banner.ts — layer order band → caption → sheen (sheen on top).
• styles.css — .banner-sheen uses mix-blend-mode: plus-lighter (fallback screen); animated spread widened to scaleX(1.11).
• *.test.ts — updated the caption pass-count and the layer-order assertion (sheen now follows caption).

Test plan

• vitest run src/content/eldenBanner.test.ts — pass
• npm run build — succeeds
• Side-by-side browser verification: reference PNG vs canvas render composited on the same dark background — overlap tone, dim echo, and brightness match
• CI green (Lint & Type Check, Build, Changeset)
• Load dist/ as an unpacked extension and confirm on a real GitHub PR

Known limitation

The reference art uses Agmena Pro (Adobe Typekit), which the extension doesn't bundle, so it falls back to Georgia — slightly different letterforms. Bundling a close free serif could narrow this in a separate change.

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