fix(grammar): accept a trailing escaped paren as a value ident in every dialect

[matthew-dean]

Problem

color: \( (and a\() is valid CSS — an escaped code point, i.e. a value identifier whose text happens to end in ( — but all four dialects rejected it:

value	before	after
color: \(	❌ all four	✅ all four
color: a\(	❌ all four	✅ all four
color: foo() / \41() / url(...) / calc(...)	✅	✅ (unchanged)

The value ident/function dispatch routed on endsWith('('), which can't tell an escaped \( from a structural (, so the token \( was misread as a function opener and the parser demanded a ). (This is the trailing-\( carve-out left open on ledger G36 by #163.)

Fix — proper dispatch, no new matcher

Each grammar's value dispatch takes a more-specific arm before the generic one:

when(endsWith('\\('), <keyword arm>),   // \( and a\( → value ident
when(endsWith('('),   <call>),          // foo( → function

Dispatch picks the first matching arm, so the escaped-paren suffix wins. It's a plain-string endsWith on the fast path — no regex, no parseman change. Applied to the value ident/function dispatch in all four grammars (css has two ladders — typed and non-typed — kept in sync). Pseudo/query/header dispatches are not value-reachable and are untouched.

The one blind spot is \\( — an escaped backslash then a real paren, a function literally named \ — which isn't valid CSS and has no reachable input; a comment records it.

Verification

• All four dialects accept \( / a\(; foo() / \41() / url() / calc() still route to their function arms. Output is byte-identical across all four (color: \( → color: \(, Keyword preserved).
• Parser suites: css 521, scss 633, less 747, jess 552 — all green.
• Pinned in test/css-superset-corpus.ts ('escaped paren in a value identifier', 'escaped delimiter in a value identifier') across all four dialect gates; acceptance matrix 6/6; less golden green.
• Closes G36's trailing-\( carve-out (now SETTLED).

Follow-up to #163; no parseman dependency change.

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