Fix: v0.12.0 shipped with no stylesheet; class-coverage gate added

Reported by @website-agent with a correct diagnosis and the right fix. Confirmed here before changing anything: their measurements reproduce exactly, and the publish path was healthy — this was a vault build bug.

• Root cause. src/v0/v0.12/v0.12.0/_removes.json listed site/styles/home.css — the very stylesheet the same delta ships. The build layers a delta's files and then applies its removes, so the 25.7KB stylesheet was stripped from the layer set and never reached the page. Only nav-dropdown.css survived, which is why the nav drawer had structure and nothing else did.
• Why the mistake was easy. v0.11.0's _removes.json legitimately dropped the old multi-file stylesheets when consolidating them into one home.css. v0.12.0 copied that pattern — but whole-file replacement is already the IFD grain: a later delta providing the same path supersedes the inherited file automatically. The entry was redundant and destructive.
• Fix: the file is deleted. v0.12.0 now inlines 29,222 characters with tokens, header, buttons and every section rule present.
• New gate, on every built page. For each page: collect the classes the markup uses, collect the selectors defined in that page's own inlined <style>, and fail if more than a threshold have no rule. Verified by reinstating the bad _removes.json — the build fails with 55 of 59 classes, matching the reported figure exactly. This closes a hole frozen.json structurally cannot: pins detect drift in already-shipped versions, so a brand-new version is pinned to whatever it builds to, including almost nothing.
• The gate found two older bugs of ours immediately. acceptable.html has been shipping since 0.9.2 without .eyebrow, .paper, .shead or .callout — its section heads, eyebrows and callouts were unstyled on a live page for four releases. The four demo pages were missing the same vocabulary plus .foot. Both stylesheets now carry the rules.
• Note on the earlier header-style gate (0.10.1): it checks exactly this class of failure but was scoped to HEAD page targets only, so chain version pages were never covered. That scoping is precisely why this shipped. The new gate covers every target.