fix(playscan): detect per-library 64-bit gaps

[snowyukitty]

Summary

• compare library basenames across each 32/64-bit ABI pair instead of stopping at directory presence
• keep whole-ABI gaps CRITICAL and report partial gaps as deterministic HIGH findings that name missing libraries
• cover matched pairs, 64-bit-only and no-native archives, mixed gaps, and the config-split exemption

Validation

• go test ./internal/playscan/
• go test ./...
• go vet ./...
• go build ./...
• go test ./internal/playscan/ -run 'TestScanArchive64BitParity/partial_library_gap_is_named' -count=100

Fixes #23