✨[Feat] Sprint 6 PNG Container Parser and Cleaner

[maruson08]

Summary

• add bounded PNG signature/chunk traversal with big-endian lengths, chunk limits, IEND/trailing handling, and compact dependency-free CRC-32 validation
• classify text, exact XMP iTXt, eXIf, iCCP, tIME, rendering/color, APNG, image, and unknown chunks
• decode PNG eXIf through the existing shared TIFF/EXIF core using the exact chunk-data view as TIFF byte zero
• add deterministic single-allocation PNG Privacy Clean and structured verification
• preserve iCCP, rendering/color chunks, IDAT, APNG structure, unknown/critical chunks, retained CRCs, ordering, and trailing bytes
• never decompress zTXt, compressed iTXt, iCCP, IDAT, or APNG payloads

Test approach

• one reusable CRC-correct PNG builder and one canonical cleaner fixture
• table-driven malformed structural cases
• exact retained-output comparison covers multiple IDAT, APNG, CRC bytes, unknown chunks, ICC, and trailing data
• offset-base, malformed eXIf removal, idempotency, determinism, input immutability, and exact subview regressions
• replaced two obsolete tests that treated PNG as unsupported

Validation

• format: pass
• lint: pass
• typecheck: pass
• tests: 206 passed in 21 files (Vitest 2.20s)
• build: pass
• package dry run: 48.5 kB packed / 251.8 kB unpacked
• npm audit: 0 vulnerabilities
• runtime dependencies: 0