Extraction hardening: jcl args multi-line over-counting

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Parent: #2516

Mode A's unbounded args search sweeps multi-line strings, creating false positives and over-counting PARM= arguments in JCL.

This needs to be addressed for JCL specifically to bound the args search appropriately, related to the broader issue in #1973.

Implementation Guide

Target File: gitgalaxy/standards/language_standards.py (JCL args key) / Core extraction logic
Problem: Because JCL is a none args-granularity language with no brace bodies, the engine uses Mode A extraction. The unbounded search can sweep a large multiline PARM= block continuously until the next structural boundary, inflating Occurrence.args counts.
Action: Either strictly anchor the args regex to the EXEC statement lines, or ensure the JCL extraction rules drop redundant hits from within a single EXEC step's parsed body.

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Fixed by #2518 (merged as 5c06080), alongside #2482 which auto-closed on merge -- this one didn't because the PR body's "Fixes #2482 and #2483" phrasing only registered the first reference as a GitHub auto-close keyword. Same fix: jcl joined _MODE_A_ARGS_CONTINUATION_MARKER so Mode A's args-search window is bounded to the step's own statement instead of the whole greedy block, fixing the documented ZOSCSEC.jcl BPXIT over-count (args read as 7, now correctly 1).