Qwen3.8: drafted-plan headroom omits the stage card's reserves #786

The single-card paths without expert tiers in PlanInputs::plan_mtp_with and plan_mtp_with_slots sum the target's terms without t.reserve_bytes, filter out CardOver violations, and derive MtpPlan::headroom_bytes from that incomplete sum. machine_for_experts adds an activation-forms reserve on integrated cards, so the returned headroom exceeds the actual headroom by those bytes. CardTotals::bytes already owns the complete sum. Use it plus the draft reserve in both paths. Correct the MTP gate's identity assertion, which currently adds t.reserve_bytes back and accepts the same error. Cover an integrated stage with a nonzero named reserve.

Evidence (at the base of #785):

• crates/model/src/arch/qwen35moe/place.rs:847
• crates/model/src/arch/qwen35moe/place.rs:964
• crates/model/src/arch/qwen35moe/place.rs:1466
• crates/placement/src/placement.rs:914
• crates/gpu-gates/src/bin/gate_qwen4exp_mtp.rs:6183

_{Filed by Shingler, the review bot, while reviewing #785; the defect is outside that pull request.}