feat(material): let every surface wear glass, not only Card - #240

Glass was a real material with one consumer. styles/glass.ts derives twenty-five --glass-* tokens from three numbers, and the only component that could read them was Card — so an app whose panels are dialogs, drawers, popovers or a navbar could set the tokens and see nothing change.

What this does

material moves into the shared vocabulary and onto Dialog.Content, Drawer.Content, Popover.Content, Menu and Navbar. solid is the default on all of them, so this adds a capability without changing a single existing render.

The rules live in _shared/material.css, keyed on [data-material="glass"] rather than a per-component class. Card had them inline, and copying that block five more times would be six places to update the next time the glass vocabulary moves — which it already did once, from thirty-one hand-declared properties to three numbers. Card now carries the attribute too and keeps its own classes, so its appearance is unchanged while the definition is shared.

Three behaviours come along because they belong to the material rather than to any one component:

• nested glass is flattened to a single pane (blurring already-blurred output is muddy rather than deeper, and costs a second pass)
• prefers-reduced-transparency falls back to an opaque fill
• so does a browser without backdrop-filter

Every --glass-* read carries a fallback. An undefined custom property makes CSS drop the whole declaration rather than fall back to an initial value, which is how a partial token set used to produce a surface with no background at all.

CardMaterial stays as a deprecated alias of Material. It is exported, it has always had exactly these two members, and removing it would break call sites for no gain.

Also: 2.5.0 could not build

*.generated.tsx is gitignored and layouts:generate only ever adds, so removing a component leaves its generated file behind, still importing the .recipe and .layout that went with it.

2.5.0 shipped with twenty-two of them, plus a twenty-third under table/. bun run check read them as components and reported "missing index.ts barrel export" for things that no longer exist; bun run build failed generating declarations. They are cleared here, and check-contracts.ts gained a stale-generated rule so the next deletion is caught at the gate with an actionable message instead of as tsc noise in a file invisible to git status.

docs/ui-usage.md also still listed Surface, GlassPanel, ScrollShadow, EmptyState and FloatingDock, all deleted. Corrected, and a Glass section added.

Verification

• bun run check — 90/90 components pass
• bun run build — clean (was failing on master)
• bun test — 85/85 pass
• docs/api-contract.md regenerated via bun run check:api -- --write

Version bumped to 2.6.0: new public API, no breaking change.