#16 feat(qwen4exp): exact 4-slot concurrent serving (stacked on #17)

Stacked on #809 (prefix cache, on #808 MTP, on #774). Merge order: #774, #808, #809, this. Review only the concurrency work and its merge onto the stack: https://github.com/dusterbloom/lucebox-hub/compare/485a8e40...15073cfa

What

Up to four concurrent sequences for Qwen3.8-Flash-Next on the sequence engine. Each slot has its own KV, recurrent and QSA indexer state. Every slot produces exactly what it would produce alone.

luce_server <model> --max-concurrency 4 --max-ctx 32768

• --max-concurrency 1 keeps the single-sequence path (MTP, prefix cache). 2..4 enables the sequence engine; more than 4 is refused. No environment variables.
• With more than one slot, MTP is switched off (--draft / --verify-width > 1 together with slots are refused) and the prefix cache stays single-slot: the sequence engine replaces the serial loop that owns both.
• Prefill chunk: auto-sized for all slots (the planner reserves the other slots' caches), capped at the single-slot target of 4,096 rows. Uncapped it chose 12,800 rows at 4x32K, which stalls the other slots' decode and peaked at 93 GB GTT.

How it stays exact

• Batched decode. A decode step is batched only when every slot is in the exactly-dense attention regime (QSA is exactly dense up to 2,051 tokens) and there are at most four rows. It runs in MMVQ batch-invariant mode, so a four-row batch uses the same kernels and arithmetic as a single row.
• Solo fallback. Any other step, and every prefill, runs each slot through the solo forward. Chunks are interleaved, so the first tokens arrive together.
• The tests follow #774's stable-QSA cache contract: the masked bucket suffix must stay finite and indexer_k's last row is the decode graph's permanent scratch row.

Tests (gfx1151, UD-Q4_K_XL, strict: bit-exact vs solo, on the full stack)

• smoke_qwen4exp_batched M 2050 3072 1: every row max_abs_delta=0 in the identical and distinct phases; untouched-slot isolation, row permutation and cancel-reset-reuse pass.
• test_qwen4exp_seq_engine M 32768: N=2/3/4 contract + soak pass, the QSA boundary crossing passes (tokens and indexer state bit-exact), distinct concurrent sequences pass.
• qwen4exp_concurrency_http.py against default flags: four parallel requests at 2.3K and 16K prompts, 8/8 identical to solo.
• test_feature_gate, test_server_unit test_qwen4exp, test_qwen4exp_mtp, test_qwen4exp_chunk, test_qwen4exp_prefix_cache pass.

Throughput (gfx1151, UD-Q4_K_XL, --max-ctx 32768, 4,096-row chunks)

	1 slot	4 parallel
prefill, 2.3K prompts	1,025 tok/s	1,096 tok/s aggregate
prefill, 16K prompts	1,179 tok/s	1,185 tok/s aggregate
decode	24.7-24.9 tok/s	6.3-6.6 per stream, 24.8 aggregate
peak GTT		89.1 GB

Above 2,051 tokens of context each slot runs its own QSA forward, so batching adds little to decode there.

Not in this PR

• Batched QSA decode, for real decode throughput above 2K context.
• Per-slot MTP and per-slot prefix snapshots.
• More than four slots.