check-env-doc is RED on main again: #2349 landed VT_QWEN35_STAGE_RESERVE_BYTES undocumented

What

scripts/agent-preflight.sh fails on a clean origin/main at 85f65b0e8:

FAIL check-env-doc
  ERROR: production env var(s) read from src/+include/ are neither documented
  in docs/ENVIRONMENT.md nor on scripts/env-doc-allowlist.txt:
    - VT_QWEN35_STAGE_RESERVE_BYTES
FAIL test_check_env_doc

Added by qwen3_5_weights.cpp:213 in #2342 / #2349.

This is the second time in two days

#2312 was the same defect one commit earlier (VT_DFLASH_BOUNDS_DEVICE, #2304). Fixing that one did not prevent this one, and it will not prevent the next.

The mechanism, which is the part worth fixing: check-env-doc runs in agent-preflight.sh, which a branch runs before it pushes. Nothing holds main itself to it — main CI is a self-healing ratchet based on the last green commit. So a change that adds an undocumented variable lands green, and the red is charged to whichever unrelated branch runs preflight next. Both times the red surfaced on a branch that had nothing to do with the variable, and both times it blocked that branch from reaching a green gate before push.

That is a base failure masquerading as a branch failure, and the cost is paid by the wrong change every time.

Fix

Document the variable, as with #2312. It is user-facing: it sets the device-memory headroom the Qwen3.5 safetensors staging decision reserves (default 12 GiB), so it changes whether weights are staged or aliased — a throughput-visible decision.

Separately worth deciding, and NOT done here: whether check-env-doc should run on main rather than only ahead of a push. Filed as a note on this issue rather than acted on, because changing what gates main is a policy change that needs its own row.

Found while gating #2314; fixed in the same flow.

Start with docs/ENVIRONMENT.md and scripts/agent-preflight.sh, then inspect scripts/env-doc-allowlist.txt and test_check_env_doc to see how production variables are recognized. Document VT_QWEN35_STAGE_RESERVE_BYTES with its default and staging behavior, and finish when check-env-doc passes on the reported main commit.