M7b: $match + $replace regex builtins

[flearc]

Summary

Implements the two remaining regex builtins on M7a's foundation, taking the regex group fully green.

• Helpers (2b6ef89): hoist is_regex + the lazy apply into shared H (third use); H.serialize now skips function-valued (and NOTHING) object keys so a raw match object serializes to valid JSON (no …"next":}); adds D3010/D3011/D3012/D3040/T1010.
• $match (7d42bc6) <s-f<s:o>n?:a<o>>: applies the regex value, walks global matches via the closure's .next, reshapes each to {match, index, groups} (jsonata's start→index), singleton-unwraps a single result, D3040 on negative limit.
• $replace (e254c4b) <s-(sf)(sf)n?:s>: string pattern → literal replace (replacement verbatim); regex pattern → a ported jsonata $-scanner ($$→$, $0→whole, $N→group via the maxDigits rule) or a function replacer (D3012 if it returns non-string); D3010 empty pattern, D3011 negative limit.
• Fidelity (63a9efe): adversarial review caught a real crash — lrexlib returns Lua false for a non-participating optional capture group where jsonata uses null, which made $replace's string replacer table.concat on a boolean and put false in $match groups. One root-cause fix (normalize false→null at the regex.lua boundary; the $N substituter only emits actual string captures) closes both symptoms and regex/case016.

Results

• Official suite 1261 → 1296 (+35, → 77.1%); regex group 10 → 39/39 (fully green), function-replace value cases +5, matchers +1.
• 528 unit tests green; zero-regression guard green (additions-only); $contains/$split and the HOFs (apply-hoist call path) byte-identical.
• Adversarial review vs genuine jsonata-js v2.2.1: $match shapes/limits, the full $N scanner incl. maxDigits + alternation/non-participating edges, function replacers, string-vs-regex patterns, errors, and the serialize fix — all oracle-faithful.

Deferred (non-suite)

$match(…)[0].groups singleton-unwrap ("b" vs ["b"]) — affects zero suite cases (a probe-only edge); the holistic cons-array navigation fix remains a separate follow-up. Plus minor cleanups (H.is_callable hoist, module-level parse_int, $split O(n²) re-slice).

Test plan

• busted spec/ — 528/0
• busted spec/jsonata_suite_spec.lua — zero-regression guard green
• bash scripts/run-suite.sh — 1296/1682, regex 39/39
• Adversarial oracle review (jsonata@2.2.1) incl. the $N maxDigits + non-participating-group edges

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