Prove the array indexing the types only asserted

[DelanoJoey]

Unblocks the gridiron integration. No behaviour change — and that claim is tested, not asserted.

What surfaced this

gridiron is the first consumer whose tsconfig sets noUncheckedIndexedAccess (marque and the rest don't), and because this package ships raw src/, the consumer typechecks our source. Eight errors appeared in concierge.tsx that no other site could see.

None are new. All are the same shape: an index read whose presence was asserted by a return type rather than proven.

	before	after
readCookie	if (!match) narrows the match, not the group; catch then re-read match[1]	check the group; catch returns the value it already had
parseRgba	declared a 4-tuple, built from m[1..4] unchecked	groups destructured and checked; a null match falls through the same branch
luminance	destructured a .map() result as a 3-tuple — it's number[], so each channel was number | undefined	one srgb() per channel, arity kept in the types

"Unchanged" is tested

Neither helper is exported, so the honest test is the observable one: does auto-theme still tell a dark page from a light one?

expect(onDark).not.toBe(onLight);

That assertion exists because the vacuous version is the trap — if parseRgba returned null for both, they'd land on the same fallback and a weaker test would pass. Mutations confirm it bites:

mutation	result
parseRgba always null	1 fail
luminance always 1	1 fail
restored	10/10

jsdom implements no hit-testing, so elementFromPoint is stubbed — the auto path samples whatever sits under the bar.

Scope

concierge.tsx only — the entry gridiron imports. article-toc.tsx and bento-grid.tsx carry the same class of error and are left for whoever first consumes them strictly; fixing them here would be unverified churn in files this PR has no reason to touch.

Verification

• suite 42/42 (2 new) · typecheck clean
• concierge.tsx clean under --strict --noUncheckedIndexedAccess
• version 1.4.0 → 1.4.1

Publishing is tag-triggered, so merging this does not release it.

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