feat(M6b): [] keepArray postfix + quoted-string step selector + trailing-; blocks

[flearc]

Summary

M6b is the parser-syntax half of the M6 split — the [] keepArray postfix plus two isolated parser fixes.

• [] keepArray — a trailing [] forces a path's final result to stay an array (suppresses the length-1 singleton unwrap). Faithful any-step rule: any step carrying [] (incl. a nested predicated sub-path) keeps the path's final result an array — e.g. Phone[type="mobile"][].number → ["077 7700 1234"] (the [] is on Phone, before .number). Implemented as a recursive path_keeps_array scan feeding the single path-branch finalize_sequence; every other finalize caller is untouched, so the singleton-unwrap default is byte-identical.
• A-2: quoted-string path step → field selector — a string after . (and in first position) selects the named field (foo."bar" ≡ foo.bar), while a standalone/predicated string ("Red"[$$="Bus"]) stays a literal. Position-based, matching jsonata.
• A-4: trailing ; in a block — ( a; b; ) is now tolerated.

Results

• Official jsonata suite: 1174 → 1211 / 1682 (+37, 69.8% → 72.5%), zero regressions. Gains across flattening (+9, incl. the deferred case041 which passes for free), quoted-selectors, object-constructor, hof-map, transform, descendent-operator, and ~10 more groups.
• 464/464 unit tests green.

Verification

Plan-grounding against the official cases caught a bug in the draft design (case037 disproved a "last-step" framing → corrected to the faithful any-step rule). An adversarial fidelity review vs the jsonata-js v2.2.1 oracle confirmed A-4 fully faithful, *[]/**[] match, and a 24-path regression sweep is clean; it found one A-2 gap (first-position quoted strings) fixed in ef953c3 (which unblocked 3 more cases).

Deferred/pre-existing (out of scope): the empty-array-vs-empty-object adapter ambiguity ($type([])→"object", a from_lua JSON-input limitation); the obscure ("x").y parenthesized-literal-head edge.

Test Plan

• busted full unit suite — 464/464
• scripts/run-suite.sh — 1211/1682, zero regressions
• Adversarial fidelity review vs jsonata-js v2.2.1 oracle

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