feat: Modernize CBACT01C COBOL batch program to Java 17

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Summary

Rewrites the COBOL batch program CBACT01C.cbl (from the CardDemo mainframe application) as a standalone Java 17 CLI application under a new java-modernization/ directory. The program reads 300-byte fixed-length account records from a flat file and produces three output files (OUTFILE, ARRYFILE, VBRCFILE), replicating the original business logic:

• Signed zoned-decimal parsing/encoding with ASCII trailing-overpunch ({/}, A-I/J-R)
• Date conversion (YYYY-MM-DD → YYYYMMDD) replacing the COBDATFT assembler routine
• Default debit substitution (2525.00 when input debit is zero)
• OCCURS array population with hard-coded slot values matching the COBOL logic
• Variable-length record split (VB1: 12-byte status, VB2: 39-byte balance summary)

Includes 25 JUnit 5 tests covering parsing, encoding round-trips, date conversion, business logic, and full end-to-end execution against the repo's 50-record acctdata.txt sample file.

A detailed MIGRATION_NOTES.md documents every COBOL-to-Java field mapping for both copybooks (CVACT01Y, CODATECN) and all three output file formats.

Review & Testing Checklist for Human

• Zoned-decimal encoding correctness: Verify the overpunch mapping in AccountRecord.parseSignedZonedDecimal() and OutputAccountRecord.formatSignedZoned() matches the COBOL ASCII zoned-decimal convention. The round-trip tests only cover a handful of values; negative inputs from real data are not represented in the sample file.
• COMP-3 fields are NOT binary-identical: The COBOL USAGE IS COMP-3 fields (debit in OUTFILE/ARRYFILE) are rendered as human-readable signed strings (e.g., +000000252500) rather than actual packed-decimal bytes. If downstream consumers expect binary COMP-3, this will need a packed encoder. See formatComp3Display().
• Field offset alignment: Confirm AccountRecord.parse() byte offsets match the CVACT01Y copybook exactly (300-byte record: 11+1+12+12+12+10+10+10+12+12+10+10+178). An off-by-one would silently corrupt fields.
• No COBOL baseline output to diff against: Tests verify structural correctness and internal consistency, but there is no actual COBOL-produced output file in the repo to compare byte-for-byte. Consider running the COBOL program on a mainframe/emulator and diffing outputs if strict parity is required.
• Test the build locally: cd java-modernization && JAVA_HOME=/path/to/jdk17 mvn clean test — all 25 tests should pass.

Notes

• The original COBOL field name typo ACCT-EXPIRAION-DATE (missing 'T') is preserved intentionally in the Java field names for traceability.
• All 50 sample records have zero debit, so the 2525.00 default substitution path is always exercised. The non-default path (copying the actual debit value) is only unit-tested, not exercised by the sample data.
• ACCT-ADDR-ZIP is parsed from the input record for completeness but is not used by any business logic, matching the original COBOL behavior.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/afe3d671c8b04d79aa65ff258f98ca37

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.