Add REXX (rexx), resolving the .cmd collision with batch (#2504)

[squid-protocol]

Adds REXX (rexx) end to end per how_to_add_a_language.md, resolving the .cmd collision with batch. Closes #2504 (parent #2516). Corpus half: squid-protocol/keyword-rosetta#138 (opened together; its own PR-triggered CI stays red until this PR merges — verify.yml checks the engine out at main — a green dispatched run against pull/<this-PR>/head is linked there).

The profile (languages/rexx.py)

z/OS TSO/E REXX (SAA) plus the ooRexx directive surface (::requires/::routine/::class/::method/::constant — .rexx files in the wild host ooRexx code). 35/53 baseline keys wired, 18 explicit Nones (every gated absence carries a validated rosetta deviation-ledger entry, rexx-stated-absences). Bare keywords use explicit guard classes ((?<![\w.!?@#$])) because REXX symbols carry ! ? @ # $ and compound-variable dots — \b cannot police them (pli's discipline).

Issue-text deviations, both #2856/#2878-style contract-over-issue rulings, pinned in the strict suite:

• ADDRESS is ipc_rpc_bridges, not io (the issue's y-clause grouped it under io): the ADDRESS statement is the host-command bridge; io's data movers are EXECIO/LINEIN(/OUTTRAP(/PULL/PUSH/QUEUE.
• SIGNAL splits three ways: SIGNAL ON → safety (handler install), SIGNAL OFF → safety_bypasses (removal), bare SIGNAL label → safety_bypasses (the GO TO unstructured-jump ruling).
• EXIT is the high_risk_execution has no stated contract: agc reads the plant while 45 languages read plant+decoy, and the rules disagree on identifiers, references, goto and deletion #2878 termination dual (high_risk_execution + panics_and_aborts), guarded against SIGNAL EXIT/CALL EXIT/EXIT: label lines.
• One owner per PARSE form: ARG → args, PULL → io, VAR|VALUE → state_mutation.

invocation_model stays default (by_name): CALL name / name() / SIGNAL name reach labels by name, so the #2866 census applies — no POSITIONAL_LANGUAGES pin.

New prism dialect family: recursive_block_rexx

REXX block comments genuinely nest (/* /* */ */ — TSO/E REXX Reference), so the language needs the recursive peel — but the shared recursive_block family also strips // line comments, and // is REXX's integer-remainder operator: the stock family would truncate real arithmetic. Following the haskell/lisp dialect precedent (#621/#770):

• delimiters ["--", "/*", "*/"] — -- is ooRexx/Regina/NetRexx's line comment; classic z/OS REXX has no line comment and adjacent -- double-negation (5--3) is vanishingly rare — the truncation trade is documented in the profile and pinned by a strict test so any future change is deliberate;
• REXX quote morphology in both nested-comment strippers' combined literal pass (gated like recursive_block_lisp's char literal): quotes double to escape, never backslash, strings can't span lines — the default single-quote branch is char-literal-shaped and would leave a /* inside 'ISPEXEC ...' un-masked for the peel loop to tear apart. Line-bounded branches keep an English apostrophe's worst-case cascade to its own line (the prism.py: _strip_nested_comments' unbounded single-quote shield swallows huge code spans (scala/rust/swift/haskell/scheme) #1302 discipline).

The .cmd collision (and why batch gained an internal_discriminator)

.cmd joins COLLISION_FREQUENCIES (batch + rexx both claim it; the hlasm .asm mechanism). rexx's discriminator: first token /* (OS/2's and z/OS's actual .cmd dispatch rule), or line-anchored PARSE ARG/PULL/SOURCE/VAR, ADDRESS <env>, SIGNAL/CALL ON|OFF, EXECIO, :: directives.

batch's rules dict is empty — it scores 0 in every Tier 3 lexical scan — so registering the collision without giving batch its own Tier 2 anchor made every real batch .cmd (and, by identity-proof weakening, .bat behavior too) resolve to rexx. batch's new discriminator uses batch-only line shapes (@echo off/on, SETLOCAL/ENDLOCAL, goto :label, no-space set NAME=, if exist/defined, %~dp0 modifiers, %ERRORLEVEL%); bare rem and spaced set = 1 are deliberately excluded — both are legal REXX assignments, and registry order checks batch's discriminator first, so a false batch hit on real REXX would lock the wrong language. Both directions are strict-suite-pinned, including a REXX file assigning to a variable named rem.

Golden-master diff (both environments, identical)

Exactly 6 diffs, all on-target for the collision machinery, zero language flips: three cpython .bat files upgrade Identity Proof from Single Indicator (Ext) to Absolute Consensus with Lock Tier 2→0, because batch's discriminator now corroborates the extension with content evidence. The corpus's two real .cmd files (powertoys batch wrappers) are byte-identical in the report — they must and do stay batch. The corpus contains no REXX sources (verified by extension sweep), so the rexx profile itself produces no crucible diff — the rosetta control shell is the semantic evidence instead.

Extraction routing

Mode A label slicing (_slice_by_labels, COBOL's paragraph slot): a routine runs from its label: to RETURN/EXIT (both already in assembly_returns) or the next label; routines don't nest. rexx joins _CLASS_START_NAMED_EXTRACTION_LANGS (::CLASS name, group 1). The args rule captures the PARSE ARG template so the per-function args counter reads the real parameter list — the groupless first draft whitespace-split group(0) to a constant 2 (the ada #6169-shape artifact the rosetta bias report caught; avg_func_args now reads 1, in-band).

Evidence

• tests/extraction/languages/test_rexx_strict.py — 97 cases; the ReDoS detonation caught a real Rule-14 adjacent-quantifier defect in the drafted compound-lvalue tails (dot inside the segment class overlapping the \. separator) — fixed before landing.
• tests/tools/language_addition_audit.py --lang rexx — all 5 invariants green.
• Full extraction suite 8311 passed; ruff_audit --ci / mypy_audit --ci baseline-clean.
• keyword-rosetta data/rexx/ — 97 gate assertions PASS; bias report unexplained cells 5→1 after landing (the survivor is the pre-existing bms/classes_found).
• Strictness row (False, False, True, False); ecosystem infra (host-command automation is the language's purpose, shell's profile).

Post-merge: rosetta PR's PR-triggered check re-run; fidelity_table.py FIDELITY_PROVENANCE re-pin owed (audit --lang warning; also owed for the prior landings).

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