Fix the contrast the audit could finally measure

Contrast is the one check I've got wrong twice, both times by being clever about
backgrounds. The first version read only background-color, so text on any of the
gradient-painted sheets was measured against transparency and the dealer puck's
dark "D" was reported at 1.2:1 against nothing. The second over-corrected and
treated low-alpha gradient stops as opaque, scoring white-on-dark at 1.0:1.

So this one is built to be incapable of false positives rather than exact.
Backgrounds composite through alpha; where a gradient is involved every colour stop
is tried and the most flattering one used. A finding means the text fails even
under the reading most generous to it.

It's checked against known answers before being believed: black on white at 21:1,
white on white at 1:1, #767676 on white at the 4.54:1 AA boundary, white over a
dark gradient, dark text on that same gradient, white over a 6% white wash on a
near-black page. Nine cases, all correct — and building them found a real bug in
the check, which walked past an opaque gradient and offered the page background as
a candidate. That made it too lenient, which is as bad as the old false positives.

What it found:

Both blind badges failed against their own gradients, in 9px bold text a dealer
reads at a glance across a table. At the worst stop: small blind 3.52:1, big blind
2.15:1. The small blind keeps its blue and darkens to 4.77:1. The big blind flips
to dark text on brighter gold, 5.84:1 — how a gold chip is marked anyway, and it
makes the two badges far easier to tell apart at speed.

Quick Split's footnote names a control the dealer is told to reach for, and that
name inherited the muted grey of the surrounding aside: 3.39:1. Emphasised text
carrying an instruction gets its own colour now.

The felt watermark came back at 1.19:1 and is meant to. It's decoration; holding it
to a reading threshold would mean making it not a watermark. It's aria-hidden now,
which says the same thing where it also does some good, and the audit skips
aria-hidden subtrees.

32 contrast passes over the table, all five overlays and Quick Split at five
viewports: zero findings, zero unmeasured. Layout audit still clean. 137 tests pass.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_015bnp7xKUPFFSpwkYNFaPgb

tests/holdem/README.md

@@ -31,7 +31,7 @@ in a browser: whether a dealer can actually reach every control.
 node tests/holdem/runUiAudit.js          # needs Playwright and the site running
 ```
 
-It checks touch-action and tap highlights, that fields stay selectable and are large
+It checks WCAG AA contrast, touch-action and tap highlights, that fields stay selectable and are large
 enough not to trigger iOS zoom, that nothing important is covered or off-screen with
 no way to scroll to it, that no grid column has collapsed to nothing, and that no
 text is painted outside its box. It runs over the table, all five overlays and Quick
@@ -43,6 +43,14 @@ this one needs a browser and a running site. Run it after any change that adds o
 resizes a control. With no Playwright to hand, paste `uiAudit.js` into devtools and
 call `LayoutAudit.runAll()`.
 
+The contrast check is built so it cannot report a false positive: backgrounds
+composite through alpha, and where a gradient is involved every colour stop is tried
+and the most flattering one used. A finding means the text fails even under the
+reading most generous to it. Two earlier attempts at this got it wrong in both
+directions — one read only `background-color` and scored text on a gradient against
+nothing, the other treated low-alpha stops as opaque — so `uiAudit.js` is checked
+against known-answer cases before it is believed.
+
 It exists because these checks were being re-typed into a console every time
 something changed, and each time they only covered whatever had just been touched.
 That is how the standings table shipped with a player-name column zero pixels wide