patches: keep the runtime-only env knobs out of the torch.compile cache key (#183)

[mhenrichsen]

From #183, point 3: @cch-zuzuche saw a cold compile (~227 s against ~150 s) every time they changed VLLM_MAMBA_ALIGN_KEEP_CHECKPOINTS or VLLM_ENGINE_STALL_SENTINEL_S.

Why

0.29's compile_factors() (envs.py) hashes every registered VLLM_* variable that isn't in ignored_factors. This repo's patches register their knobs so they can be read through envs, and #114 required that for good reason: the spec-decode ones really do change compiled code. But registering a knob that no compiled graph reads puts it in the key anyway. Measured on the installed 0.29 tree:

• VLLM_ENGINE_STALL_SENTINEL_S: read by the engine loop's watchdog thread (v1/engine/core.py). Registered, not ignored.
• VLLM_MAMBA_ALIGN_KEEP_CHECKPOINTS: read by the KV block manager (v1/core/single_type_kv_cache_manager.py). Registered, not ignored.
• VLLM_DFLASH2_CHAIN_LOG_SEC: a log interval.
• VLLM_MARLIN_TUNE_DIR: a directory path that nothing reads.
• VLLM_PREFIX_CACHE_RETENTION_INTERVAL: the deprecated env spelling of --prefix-cache-retention-interval, which CacheConfig.compute_hash already leaves out of its own hash for exactly this reason. The launchers use the flag, which is fine, but the env var still cost a cold compile.

The change

A new last entry in patches/series, compile-key-runtime-knobs.patch, adds those five names to ignored_factors beside upstream's own runtime-only entries. It's a separate patch so the fork-exported files stay untouched; it's hand-cut against 0.29.0 with the series applied, and PATCHES.md says so.

Verification

• check_vllm_series.sh on pristine v0.29.0: 43 at exact context, 0 offset, 0 fuzz.
• compile_factors() on the reference box's installed 0.29 envs.py, before and after the patch, hashing the key under each setting against the baseline:

setting	before	after
VLLM_ENGINE_STALL_SENTINEL_S=300	key changes	same key
VLLM_MAMBA_ALIGN_KEEP_CHECKPOINTS=1	key changes	same key
VLLM_DFLASH2_CHAIN_LOG_SEC=5	key changes	same key
VLLM_PREFIX_CACHE_RETENTION_INTERVAL=13056	key changes	same key
control VLLM_SPEC_DECODE_ATTN=1	key changes	key changes
control KVARN_SPLIT_K=8	key changes	key changes
control VLLM_DFLASH2_LOOKUP=1	key changes	key changes

The controls are knobs that do reach compiled code, and they still invalidate the cache. So the patch removes exactly the five names and nothing else.