feature: add COBOL-to-Python migration plan (MBA-1423)

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Summary

Documentation-only. Adds documentation/technical/python-migration-plan.md and makes Python a first-class translation target in the README, closing MBA-1423.

The plan is built from a measured inventory of the tree rather than generic advice — 38 programs / 20 copybooks, with 8 programs issuing EXEC CICS, 14 issuing EXEC SQL, 16 declaring KSDS files, and 4 copybooks carrying COMP-3 fields. Those counts drive a 4-tier scope split (no-dependency / KSDS / DB2 / CICS) and a 6-wave sequence where each wave has an explicit exit criterion.

Two decisions worth flagging for review:

• Fixed-point is treated as the primary correctness risk, not CICS/VSAM. PIC S9(13)V9(2) COMP-3 has no Python analogue, so the plan mandates Decimal with an explicit quantize() on every store, and puts the pilot on numeric validation before any volume translation.
• Translations are paragraph-preserving, not idiomatic. 1000-VALIDATE-ID becomes _1000_validate_id so a reviewer can diff Python against COBOL paragraph-by-paragraph. Readability is deliberately traded for traceability, since this is benchmark reference material.

Wave 0 (the "first migration step" acceptance criterion) is specified down to the file list: the Picture/record-codec runtime, PORTVAL.cpy + PORTVALD translated, and a differential pytest that runs both the GnuCOBOL-compiled module and the Python function over one input corpus and asserts equal LS-RETURN-CODE / LS-ERROR-MSG. PORTVALD was chosen because an executable oracle exists today — verified locally:

cobc -m -I src/copybook/common -o build/PORTVALD.so src/programs/portfolio/PORTVALD.cbl   # compiles clean

No COBOL source is touched; the plan explicitly prohibits editing it to ease translation, which would invalidate the benchmark.

Related: PR #94 added a ticket stub for this work and also edits the README Future Plans line, so the two will conflict on that line if both land.

Link to Devin session: https://app.devin.ai/sessions/273b91ab7f46499c8b1e5b652145b733

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