fix(dashboard): validate locale parity input - #7497

Changes

• Flatten locale arrays with explicit indexed paths while keeping empty arrays distinct from scalar values.
• Keep array indices distinct from numeric object keys so malformed locale shapes cannot collide.
• Reject malformed non-object locale roots and wrap read or JSON parse failures with the affected filename.
• Add importable script helpers, focused regressions, and a changelog fragment.

Verification

• mise exec -- pnpm --dir crates/librefang-api/dashboard lint
• mise exec -- pnpm --dir crates/librefang-api/dashboard typecheck
• mise exec -- pnpm --dir crates/librefang-api/dashboard test (102 files, 1078 tests)
• mise exec -- pnpm --dir crates/librefang-api/dashboard build
• python3 scripts/check-changelog-attribution.py --base origin/main --head HEAD
• git diff --check origin/main...HEAD

Out of scope

• Locale content and the CLDR-aware parity policy are unchanged.
• Locale-directory discovery remains rooted beside the checked-in script.
• Existing Polish and Ukrainian plural-key drift remains unchanged.