Theme-agnostic DocsSite stack; one-line EE branding

[russwyte]

Why

DocsSite.standardLayers hardcoded Theme.default among nine layers, so any branded consumer had to re-list the entire ZLayer.make block. That block is copy-pasted verbatim in five sites across the org (specular, zipx, sbt-dynver-ci, ascent, the hub) — which is where doc-theme drift actually comes from.

What

• DocsSite.themedStack: ZLayer[Theme, Nothing, SiteBuilder] — the same stack via ZLayer.makeSome, with Theme left as an environment hole. Nothing in the stack provides Theme (it is consumed by PageTemplate.live, LandingTemplate.live and SiteBuilder.Env), so this is a hole standardLayers merely happened to plug.
• DocsSite.standardLayers = Theme.default >>> themedStack — unchanged meaning for existing callers.
• EarlyEffectTheme.layers — pre-composed branded stack.
• EarlyEffectTheme.brand(site) — sets logo / logoLink, using orElse so an explicit value still wins.
• Dogfood BuildSite and the prose in README / Concepts / LibraryAuthors updated to the new three-one-liner shape.

Consumers become:

override def site   = EarlyEffectTheme.brand(super.site)
override def layers = EarlyEffectTheme.layers
override def afterBuild(out: Path, result: SiteOutput) = EarlyEffectTheme.writeLogo(out)

Verification

• sbt testFull green; two new tests cover the split — standardLayers still emits stock ThemeTokens.default CSS with no prefers-color-scheme block, and themedStack picks up a caller-supplied theme instead.
• sbt scalafmtCheckAll green.
• Built the dogfood site before and after: diff -rq shows only concepts.html, library-authors.html and assets/client.js differ, all from the prose edits. assets/theme.css, index.html and metadata.json are byte-identical.