fix: support legacy v18-v20 ACD layouts

[Bobby-Miller]

Summary

• parse the four-byte-short legacy FAFA/FDFD Comps headers and legacy Rx envelope without changing generated modern parsers
• recover legacy Region Maps, data-type graphs, arrays, motion/message tags, aliases, Produced/Consumed connection metadata, and task schedules
• preserve modern Rx/Region Map detection, including header-only empty maps, and add the observed 1769-L33ERM catalog mapping
• reject malformed legacy lengths, aliases, connection descriptors, and task program counts instead of silently returning partial data

Validation

• 94 passed, 2 skipped on Python 3.8
• 94 passed, 2 skipped on the current Python interpreter
• repository CI flake8 error gate: 0 findings
• synthetic coverage for modern/legacy Comps, Rx, Region Map collision handling, scalar/array/UDT/motion/message tags, aliases, Produced/Consumed links, and task schedules
• validated five private v18/v20 projects against Studio-generated v37 L5X references: exact tag scope/type/data type/dimensions/radix/access, aliases, ProduceInfo/ConsumeInfo, and task schedules; all generated L5X files are well-formed

The proprietary validation projects are not included in this PR.