test: Add 20 function-library fixtures incl. repr_* round-trips

[leongdl]

Summary

20 function-library fixtures from a coverage audit of RFC 0006. Suite: EXPR 350 → 368 passing, 0 failing (2 new fixtures are windows-gated and skip on POSIX).

Areas covered

• repr_* round-trips — the security-critical surface. The audit found all five pre-existing repr_* fixtures assert the quoted text as a substring; none executed it. Quoting is only correct if the target interpreter agrees, so these fixtures pipe the quoted output through the real target: repr_sh through bash (15 adversarial payloads: metacharacters, $(...), backtick, nested quotes, backslash, glob, .., leading -, unicode, empty, whitespace-only), repr_py through ast.literal_eval (16 payloads), repr_json through json.loads (15 payloads) — each asserted byte-identical after the round trip.
• repr_cmd/repr_pwsh round-trips: written and windows-gated (runOn: [windows]); they skip on POSIX and need a Windows CI run to count. %VAR%/!VAR! payloads excluded from the cmd battery per the spec's own .bat-context scoping.
• Unicode codepoint semantics (the audit verified zero non-ASCII data existed anywhere in the EXPR suite): len on CJK/emoji/combining-mark/astral strings; case transforms restricted to 1:1 mappings; isascii/isalpha classification; padding to codepoint widths.
• int64 inside functions: abs(-2^63) rejected (negation overflow), sum overflow rejected, floor(1e300)/ceil(1e300) rejected, int() at the exact-double 9e18 boundary.
• Predicate/conversion edges: isupper("123")/islower("123") false (the "no cased characters" clause), predicates on "", bool("TRUE")/bool("1") case-insensitive forms, range_expr(" ") rejected.
• Regex-dialect pins (divergence probes): lookahead, backreferences, and group references in re_sub replacements are all rejected by openjd-rs exactly as §2.2.5 requires — but Python's re natively supports all three, so a naive Python-backed implementation would accept them. These three .invalid fixtures now pin the conservative dialect for every implementation.

Deliberately excluded

~160 arity/type error cases (indistinguishable under the current runner, which asserts only a non-zero exit) and ~78 null-argument cases (the spec defines null behaviour for only 6 of 84 functions — a spec decision is needed first). Writing these now would manufacture confidence.

Companion PR

Three parked fixtures with dual-implementation observations (both-implementation repr_py newline bug; center()/isdigit() spec gaps where both implementations agree) are in the expected-failures PR (branch conformance-func-lib-expected-failures).