Let tail chunks co-batch with cache-hit requests (SGLANG_CP_PREFILL_MIX_CHUNKED)

专题 S1 (design: docs_internal/perf/prefill-compute-intensity-plan.md
S1.0-S1.11).  A batch containing a chunked prefill request has been
forced to bs=1 by the CP gate, so every chunk of a long prompt
monopolizes a forward while short-extend cache-hit continuations queue
— the direct cause of the replay TTFT tail (p90 19.3s / p99 49.2s at
91.8% cache hit).  Yet mixed chunk batches already occur today (a
freshly-chunked request keeps earlier-admitted small requests), proving
the CP forward path is mixed-chunk-safe; only admission was asymmetric.

Three changes, the first flag-independent:
- add_chunked_req now seeds the budget with the chunk's TRUE prefix
  (was 0), so the CP cached tally and the buffer estimator's mqa_logits
  k_rows see the chunk's footprint before any later request is admitted
  (landmine D1).
- New SGLANG_CP_PREFILL_MIX_CHUNKED (default OFF): with it on, the gate
  admits requests after a chunked one and lets the existing CP caps
  (extend / cached / buffer, now correctly seeded) bound the batch — a
  FULL chunk still ends the scan by consuming the chunk-clamped extend
  cap; only a tail chunk leaves headroom.  A chunked prefix that is not
  page-aligned (rare sub-page final-chunk tail) keeps its batch solo
  (the CP page-aligned split would fail-fast otherwise).
- The symm staging capacity identity (admission extend cap + request
  slack == staging pages) is asserted when the flag is on, locking the
  coupling the design relies on (plan doc S1.4 I2).

Tests: 4 new adder units (budget seeding; tail chunk admits followers;
full chunk solo by budget; non-aligned prefix solo); the 8-rank
byte-exactness scenario gains a chunk-shaped request (2048-token
page-aligned carried prefix + 512 extend) — all four phases
(legacy/v2/symm/prefetch) byte-identical on g0033.  Known pre-existing
cross-file pollution noted in problems.md P16.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

python/sglang/srt/environ.py

@@ -236,6 +236,12 @@ class Envs:
     # Total symm slab size override in MB (both halves). 0 = pool-derived
     # hard bound (logical pages x dense page unit, see design doc).
     SGLANG_CP_SHARED_KV_SYMM_HEAP_MB = EnvInt(0)
+    # Allow a chunked prefill request (page-aligned carried prefix) to
+    # co-batch with other requests under CP shared-KV bs>1: a tail chunk
+    # leaves extend-cap headroom for short-extend cache-hit requests; a full
+    # chunk still ends the scan by budget.  See
+    # docs_internal/perf/prefill-compute-intensity-plan.md S1.
+    SGLANG_CP_PREFILL_MIX_CHUNKED = EnvBool(False)
     # NSA MQA logits are materialized as fp32 [q, k] buffers inside DeepGEMM.
     # Lower values split query rows more aggressively to cap peak temporary memory.
     SGLANG_NSA_MQA_LOGITS_FREE_MEM_FRACTION = EnvFloat(0.2)