+ add full NuFX ShrinkIt archive read-write support - #128

What changed

Adds genuine NuFX / ShrinkIt archive support for Apple II and Apple IIgs archives. This PR contains the corrected NuLZW commit from #126 plus one NuFX container commit; after #126 merges, the diff naturally reduces to the NuFX-only commit.

Read / validation

• parses the 48-byte master header and v0/v1/v2/v3 record/thread layouts
• validates master CRC, record-header CRC and v3 uncompressed thread CRC-16/XMODEM
• supports filename threads and deprecated in-header filenames, Mac Roman names and filesystem separators
• extracts Stored, native headerless Squeeze, NuLZW/1 and NuLZW/2 file/disk threads
• recognizes LZC-12/LZC-16 structurally without falsely claiming a decoder
• reads plain SHK/SDK and locates Binary-II-wrapped BXY NuFX data; wrapped archives are intentionally read-only for direct mutation
• header/structure/integrity validator support

Squeeze interoperability

NuFX format-1 threads are not standalone .SQ files. They omit the 76 FF/checksum/filename header and begin directly with the node table. This PR therefore includes a dedicated NuFX Squeeze path with:

• historical 0x90 RLE (value, 0x90, count; delimiter escaped as 0x90,0)
• serialized signed-child Huffman node table rooted at node 0
• LSB-first Huffman codes and EOF symbol 256
• zero-node empty-stream representation
• guard-byte behavior compatible with read-ahead decoders
• a byte-level regression test that fails if a NuFX thread accidentally contains the standalone 76 FF header

Creation

• normal multi-record .shk
• single disk-image .sdk with 512-byte block-count/storage semantics
• Stored, Squeeze, NuLZW/1, NuLZW/2 and Auto-smallest methods
• version-3 CRCs and canonical filename threads with ShrinkIt-style reserve
• ProDOS file type, aux type and access options
• rejects password/encryption and unsafe paths

Genuine direct R/W

CanModify is backed by byte-level record editing:

• append inserts a complete record at current master EOF
• replace rewrites only the selected record extent and shifts the following raw tail
• remove closes only the record extent and clears the truncated physical tail
• every individual edit immediately patches pre-edit record_count ± n, exact master EOF and master CRC, so a following edit never reparses stale metadata
• replace preserves filename buffers, resource forks, comments, option lists and unknown threads byte-for-byte; only the selected data thread is re-encoded
• records missing a data fork can gain one without discarding their existing threads
• rare v2 direct replacement is refused because its thread CRC semantics differ; safe raw append/remove remain available

Maintenance

• defrag compacts documented oversized filename/comment reserves while copying data/unknown threads raw
• shrink uses the same content-preserving compaction and only emits it when actually smaller
• archive layout map exposes master + real record extents

Tests

All writable methods, nested/Mac-Roman names, SDK, Squeeze wire representation/RLE edges, sequential multi-add master bookkeeping, byte-preserving tail replacement, removal, shrink/defrag payload preservation, CRC validation, encryption rejection and unsafe paths.

Depends on #126 for NuLzwCodec.