A match arm `case PAT: (expr)` — e.g. `case 0: (5)` — failed to parse:
parseMatchBody unconditionally consumed an `(` after `case PAT:` as a
payload-capture `(ident)`, so a non-identifier first token produced
"expected capture name".
Disambiguate: treat `(` as a capture only when it encloses exactly a lone
identifier — `( ident )` — via a new isLoneIdentParen() helper (peekTag-based
two-token lookahead). Otherwise the parens belong to the arm-body expression.
Payload capture (`case .b: (v) { ... }`, examples/128) still binds.
This fixes the scalar paren arm value (`case 0: (5)` now parses and runs).
The tuple arm-value form (`case .X: (a, b)`) additionally needs a tuple
literal in statement/binding position, tracked separately as issue 0059.
Tests: two inline parser unit tests (paren arm value is not a capture; lone
`(ident)` still binds). Gates: zig build, zig build test, 273/273 examples.
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