feat(core): add stable fingerprints to findings and machine-readable output

[Krishan27]

Implementation of ADR 007. Adds fingerprint field to FlagUsage. Same finding produces same fingerprint across runs. Line-number changes do not change fingerprint. JSON output includes fingerprint.

[Krishan27]

Closed by PR #175 (feat/stable-fingerprints merged to main). Every finding now carries a stable fingerprint field in scanner output and JSON reports.