[noahgift]

Closes #4445

apr bench on a Qwen3.5 GGUF refuses every run with warmup+iterations > 1: warmup and each timed turn repeat one prompt on one Session, the session resumes the repeated prefix, the turn reports reused != 0, and the prefill guard (correctly) refuses.

Fix. Session::forget_prefix() drops every prefix the session could resume from. bench_qwen35 calls it before each warmup turn and in session_timed_turn, so every timed turn prefills its whole prompt.

Opened against main ahead of car #4429 (operator: "make it faster"). On main, Session has no #4214 checkpoint, so forget_prefix clears processed only. It destructures Session exhaustively, so when the car's checkpoint field meets this code the merge fails to compile until the checkpoint is reset too. It cannot merge silently with a resumable prefix left behind. The car-side version (clears the checkpoint too; its test goes red on a forget_prefix that keeps the checkpoint, measured) is 9c34ca648d on bb/4445-bench-fresh-prefill, quorum 3/3 on that branch. Whichever order lands second takes that one-line resolution.

Tests.

• forget_prefix_makes_an_extending_prompt_prefill_whole_again: on main an extending prompt normally resumes (reused == 2). After forget_prefix it must prefill from 0.
• session:: 31/31 pass on this branch.

🤖 Generated with Claude Code

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-4445",
 "head": "4877908462588896d4e313d1ba2fbc44610dba3f",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 2
  },
  {
   "lane": 2,
   "verdict": "FAIL",
   "findings": 3
  },
  {
   "lane": 3,
   "verdict": "FAIL",
   "findings": 1
  }
 ]
}

[noahgift]

quorum-review (AD-04): three PASS — agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-4445",
 "head": "874891dfe80afe9c0409c1b3beda4071d1713455",
 "width": 3,
 "executor": "agy",
 "agreed": true,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=4577 head=51c41b92e2ee6bc54867fd87885632a842ecc040 verdict=REFUSE class=Q1 arm_rc=1

Shadow mode: this records a verdict and merges nothing. A refusal
to arm is not a block (§13 adds zero rows to §7) — the pull request is
exactly as green as it was.

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-4445",
 "head": "51c41b92e2ee6bc54867fd87885632a842ecc040",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "NO-VERDICT",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): three PASS — agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-4445",
 "head": "51c41b92e2ee6bc54867fd87885632a842ecc040",
 "width": 3,
 "executor": "agy",
 "agreed": true,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

Degraded quorum (PMAT-360). same-family (policy: agy reserved for tier-1 diffs) — withheld by policy: gemini (policy); Claude Code lanes: 2. Armed under the default of the operator, verbatim: "we are STILL draining agy too quick. we need to switch default to sonnet, agy, haiku", with agy reserved for tier-1 diffs (quorum.agy_tier1_only, cop ruling 2026-09-24). This is a same-family verdict, not an independent one.