feat: migrate _composite.py + _segment.py producers to pyx12 codes (PR 3 of 6)

[azoner]

Third slice of the X12 error code generalization plan. Composite and segment validators now emit pyx12 codes; visitors translate via the ERROR_CODES table so 999/997 ack output is unchanged.

Producer migration

pyx12/map_if/_composite.py (3 sites):

• Mandatory composite missing: \"1\" → COMP_1_MANDATORY_MISSING
• Composite marked Not Used: \"5\" → COMP_5_NOT_USED
• Too many sub-elements: \"3\" → COMP_3_TOO_MANY_SUBELEMENTS

pyx12/map_if/_segment.py (3 sites):

• Too many elements in segment: \"3\" → SEG_3_TOO_MANY_ELEMENTS
• Too many sub-elements in composite (segment-level check): \"3\" → SEG_3_TOO_MANY_SUBELEMENTS
• Syntax violations: \"10\" → SEG_10_SYNTAX_EXCLUSIVE, \"2\" → SEG_2_SYNTAX_RELATIONAL

Test updates

4 test_map_if assertions updated to expect pyx12 codes:

• test_segment_length → SEG_3_too_many_elements
• test_comp_required_fail1 → COMP_1_mandatory_missing
• test_comp_not_used_fail1 → COMP_5_not_used
• test_plb03_composite_missing → COMP_1_mandatory_missing

resJson fixtures

Unchanged. Re-running the regenerator helper made no diff, confirming no existing resJson fixture exercised composite-level or segment-validator-level error paths. The 39 remaining raw X12 codes in resJson are all from walker/parser emissions, which migrate in PR 4.

Test plan

• pytest pyx12/test/: 579 passed (no count change)
• mypy --strict pyx12: clean (88 files)
• ruff check + format --check: clean
• res997/resAck unchanged
• resJson unchanged for these paths (no-op regenerator confirms)

Plan reference

~/.claude/plans/radiant-petting-steele.md — see PR 3 section.

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