dec: core security hardening against memory validation and resource flaws

[owvr27]

Summary of Hardening Changes

This unified patch introduces production-grade defensive validations across the decoder core (c/dec/decode.c, c/dec/huffman.c, and c/dec/state.c) to eliminate five distinct resilience and memory boundaries gaps surfaced via dense stream state anomalies:

1. Huffman Table Budget Safeguard: Introduces an explicit table_budget constraint parameter inside BrotliBuildHuffmanTable to halt multi-level accumulation before breaching array structures, accompanied by a strict trailing slab_end range-check inside HuffmanTreeGroupDecode to insulate against heap OOB writes.
2. RingBuffer Integer Overflow Mitigation: Shifts BrotliCalculateRingBufferSize calculations to precise unsigned arithmetic to completely eliminate undefined behavior (UB) on large window configurations (e.g., window_bits = 30), forcing saturation thresholds at window_size to block unconstrained 1 GiB OOM allocation logic vectors.
3. Cold-Start Heap Disclosure Prevention: Enforces strict execution limits on speculative 16-byte copy macro activities inside the LZ77 backward-copy operations, introducing a precise exact-length loop fallback when rb_roundtrips == 0 to safeguard uninitialized heap buffers from flushing to the output context.
4. State Machine Desynchronisation Isolation: Explicitly resets s->loop_counter upon transitioning into BROTLI_STATE_METABLOCK_DONE states, cutting off intermediate state leakages during streaming boundary suspensions, backed by a defensive invariant verification BROTLI_DCHECK at function entries.
5. Resource-Asymmetry DoS Mitigation: Restricts maximum incoming num_literal_htrees values against logically reachable context mapping indices, and deploys global multiplication overflow validation protections prior to executing internal memory allocation macros within BrotliDecoderHuffmanTreeGroupInit.

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Verification

All modifications maintain full API integration limits and seamlessly route violations into existing error blocks (BROTLI_DECODER_ERROR_FORMAT_HUFFMAN_SPACE, etc.) without introducing custom public API disruptions. Structural verification confirms no degradation against standard high-performance decoding hot paths (s->rb_roundtrips > 0).