test(huf): targeted regression tests for HUF 4-stream burst gate boundary states in decode_literals

[coderabbitai]

Summary

Add targeted unit tests for the HUF 4-stream burst gate boundaries in literals_section_decoder.rs::decode_literals (lines 148–292).

Motivation

The existing decode_corpus / encode_corpus / level_1_fast c_stream integration tests cover the burst gate states organically, so a regression would surface before merge. However, explicit unit tests would significantly speed up triage on any future gate tweak.

This work was identified as out of scope for #201 (requires ~200 LOC of synthetic HUF bitstream scaffolding) and is tracked here as a follow-up.

Edge cases to cover

1. bits_consumed == max_num_bits — exact lower boundary of the burst gate; the burst is entered for the first time (gate condition is met with zero slack).
2. bits_consumed + burst_bits == 64 — exact upper boundary; the burst consumes all remaining bits in the 64-bit window without overflow.
3. SIMD-fallback → refill → burst re-entry transition — the outer loop falls back to the SIMD 4-symbol path, a BitReaderReversed refill occurs, and the next iteration correctly re-enters the burst path once the gate holds again.

Acceptance criteria

• Synthetic HUF bitstream scaffolding (~200 LOC) that can construct streams hitting the three states above deterministically.
• Unit tests asserting correct symbol output and correct bits_consumed / bit_container state after each transition.
• Tests are gated under #[cfg(test)] and run cleanly under cargo nextest.
• No ratio regression; clippy clean.

References

• PR: perf(decode): port donor HUF 4-stream burst with sentinel-bit ctz #201 (port of donor HUF 4-stream burst with sentinel-bit ctz)
• Comment: perf(decode): port donor HUF 4-stream burst with sentinel-bit ctz #201
• Requested by: @polaz

[coderabbitai]

🔗 Related PRs

#30 - feat: large literals block support (>262KB) [merged]
#54 - perf(decoding): 4-stream interleaved Huffman decode and bulk table init [merged]
#92 - perf(decoding): SIMD HUF kernels with runtime dispatch [merged]
#182 - feat(encoder): strategy-aware literal gates (G4 + G5) [merged]
#194 - perf(decode): fused sequence executor + SIMD/FSE hot-path cleanup + DoS-safe rollback [merged]

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[polaz]

Folded into #201 via 1bc35db — burst-gate regression tests now live in zstd/src/decoding/literals_section_decoder.rs::burst_gate_tests. All 4 tests pass; full nextest suite (538/538) green. Closing on merge.