Add HTTP/2 HPACK (RFC 7541) and LHA -lh2- codecs

[MagicalTux]

Adds the two requested codecs so the crate covers "h2 huffman" and LHA -lh2-.

HTTP/2 HPACK — new hpack feature (compcol::hpack)

Full RFC 7541 header compression, not just the Huffman primitive:

• Integer coding (§5.1) N-bit-prefix, with overflow/overlong guards.
• String literals (§5.2) raw or Huffman, with EOS-padding validation.
• Static table (Appendix A, 61 entries) + dynamic table with §4.1 byte
  accounting, §4.4 eviction, and §6.3 size updates.
• All field representations: §6.1 indexed, §6.2.1 literal+incremental
  indexing, §6.2.2 without indexing, §6.2.3 never-indexed, §6.3 size update.
• HpackEncoder / HpackDecoder hold dynamic-table state across header
  blocks. HPACK works on (name, value) header lists with per-connection
  state — not a byte stream — so it gets its own API rather than the
  Encoder/Decoder traits.
• The §5.2 string Huffman code (Appendix B) is also exposed as the
  Http2Huffman codec (name h2-huffman) on the uniform trait surface and is
  registered in the factory.

Validation: byte-for-byte against the RFC 7541 Appendix C worked examples —
C.1 integers, C.3 (raw) and C.4 (Huffman) request sequences including dynamic
table evolution, plus the C.4 string vectors. Decoder error paths (index 0,
bad index, oversized size update, decoded EOS, bad Huffman padding, truncation)
are covered, plus a decoder_h2_huffman fuzz target.

LHA -lh2- — extends the lha feature

8 KiB-window LZSS with adaptive (dynamic) Huffman for both literals/lengths
and match positions — the dynamic position tree is what separates lh2 from
lh1's fixed position code. The adaptive-tree machinery is generalized to a
runtime alphabet size shared by the 510-symbol char tree and the 14-symbol
position-class tree. Like lh1 it is continuous/size-terminated, so its decoder
takes the length via DecoderConfig::with_len. Registered in the factory.

Validation: clean-room from the public dynamic-Huffman description;
round-trip validated (empty/small/incompressible/repetitive/binary, plus a
far-window match). There is no public -lh2- reference fixture (no mainstream
tool emits the method), so bit-exact interop with archives in the wild is
best-effort — documented in the module and matching how the other LHA methods
are validated ("own round-trip, no reference fixture").

Checks

• cargo build --all-features and no_std (--no-default-features --features hpack / --features lha) both clean.
• cargo test --all-features green (all suites, incl. 17 hpack unit + 5 hpack
  integration + lh2 round-trip/refusal/large-window tests).
• cargo clippy --all-features — no warnings.

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