feat: tell the pack which ground the card is painted on (#107)

[lex00]

Closes #107.

A pack's colored glyph is spliced onto a card whose fill pinhole itself
resolved, and iconFor(kind) was handed nothing about that fill. behold's Helm
mark (behold#246/#255) could only fix its 1.01:1 contrast by baking a plate that
works on every ground — chosen blind. Every pack shipping brand artwork would
re-solve it the same way.

The contract

iconFor gains an optional second argument:

iconFor(kind: string, ctx?: IconContext): string | GlyphSpec | undefined;

ctx.ground is the literal fill attribute value the painter is about to write
on the shape directly under the glyph, in pinhole's usual
var(--pin-<token>, <baked>) form — var(--pin-warnFill, #2A1417) for a
_status: "warn" card in the dark theme.

That form was picked over the alternatives because both halves carry weight: the
token name says which ground pinhole chose (i.e. the status), and the baked hex
says what it actually looks like in the theme being rendered, which is what a
pack needs to measure its own ink against. The token name alone leaves a pack
unable to do contrast math (it doesn't know the theme); the hex alone hides
which status was chosen and lies about live --pin-* overrides. The var
expression is also verifiably truthful — it's the exact string that lands in the
rect, and the tests assert the two by parsing the rendered SVG.

ground is undefined where pinhole has no honest single answer: the
containment views paint boxes and badges at fill-opacity/group opacity below
1, so the glyph reads against a composite over the page background. A pack
seeing undefined is exactly where it was before this change.

Threading

resolveGlyph(node, { override?, ground? }) passes it through; an override still
short-circuits the pack, so no ground is offered when the icon was overridden. A
new exported statusGround(status, theme) in paint/svg.ts is the single place
the value is derived, used by the card painter and by the flow / stacked /
small-multiples / morph views, so it can't drift from what those painters write
into the rect.

Compatibility

Additive. The parameter is optional, so iconFor: (kind) => … still satisfies
PresentationPack (a test asserts it as a typed literal) and resolves the same
glyph whether or not a ground is supplied. The bundled gitlab and aws packs are
untouched. behold is untouched — this only makes the contract capable of the
conditional plate.

Gates

npm run tsc, npm test (195 → 206), npm run build all green.

🤖 Generated with Claude Code