# Recommendation — Phase C+23

## Top-line: STAY WITH THE BAND-AID

After source-reading both engines + characterizing 4 archived canary
cold runs' jitter shape + reviewing Phase D's H'/H broad outcomes,
the recommended approach is **(ζ) stay with the band-aid**.

The 104,607 cap that originally motivated this track is already
unblocked at the diff-tool layer (Phase D D-extension absorber,
2026-05-18). The next divergence at idx 105,046 is
`VdInitializeEngines.return_value` — a VD-subsystem engine bug, NOT
a scheduling-determinism recurrence. The cost-benefit of pursuing
γ/β/α is no longer compelling because the immediate symptom is
resolved and no structural follow-on cap has appeared.

## Rationale

### 1. The original target is already unblocked.

| metric | pre-C+20 (C+19) | post-C+21 | post-Phase-D D-extension | now |
|---|---|---|---|---|
| Main matched-prefix | 104,606 | 104,607 | **105,046** | 105,046 |
| Sister chains | 11/32/3/41/16 | 11/32/3/41/16 | 11/32/4/41/16 | unchanged |
| Cap class at head | (B) contention | (A) state-mutation | (engine) VD | (engine) VD |

The matched-prefix advanced **+440** since C+19 through diff-tool work
that did NOT touch the engines. The cap class at the head is no longer
scheduling.

### 2. Phase D Stages 1-4 already built the structural infrastructure.

Phase D Stage 1 (canary contention emitter), Stage 2 (manifest builder),
Stage 3 (ours `OrderMode::ContentionReplay` + manifest loader), and
Stage 4 (diff-tool engine-local kinds) ALL LANDED. The engine code is
in tree. What's missing is *coverage of the right contention events*:
the 104,607 divergence was upstream of canary's first
`contention.observed=true` emit (idx 104,664), so the manifest could
not target the right call site.

This means: if we pursue γ (broaden replay to more event classes),
the entry cost is not "start from scratch" but "extend an existing
manifest layer." However, the LOC budget for γ is still ~600 across
both engines, and there is **no proven future cap** that this would
unblock.

### 3. The empirical jitter range is small and fully absorbable.

From `jitter-profile.md`: 4 canary cold samples show 3 distinct
shapes around the contention window. The C+21 absorber + Phase D
D-extension already canonicalize ALL 3 shapes to the same matched
form. Even N=5 or N=10 fresh canary colds would land in one of these
3 shapes (likely with the same absorber outcome).

The SID core (`a25a16a4f6f547aa`, `2a70efeeed4f4fb6`,
`72a4170012353517`) is consistent across cold runs (±20% counts), and
the shared-global SID recipe (C+18) recomputes them deterministically.
The transient "top-2" SIDs (which change per-cold) all flow through
the shared-global absorber.

### 4. Canary cannot be made deterministic without invalidating it.

The host-thread-per-XThread model is what makes canary the *oracle*.
Replacing it (α / β) would require:

- Reworking ~2000-3000 LOC of canary base+kernel.
- Re-validating against the broader canary test corpus (other games).
- Accepting a real risk of breaking Sylpheed-unrelated game-compat.

Approach γ (record-and-replay) avoids touching canary's scheduling
philosophy but requires ours to consume a multi-million-entry trace,
with engineering and runtime cost that should be matched to a *proven*
future scheduling cap.

### 5. The Phase B image hash and ours digest are stable.

`image_loaded_sha256 ea8d160e…` UNCHANGED. Ours default digest
stable × 3 cold runs. There is no signal of latent divergence in the
pre-Phase-A surfaces that would benefit from scheduling alignment.

## What to keep

1. **Phase D Stages 1-4 infrastructure** stays in tree. Cvar
   `kernel_emit_contention=false` default-off; `XENIA_CONTENTION_MANIFEST_PATH`
   opt-in. Future phases can use them.
2. **All absorbers** (C+18, C+21, D-extension) stay; they are correct
   and narrow.
3. **The Stage 0 `OrderMode::ScanQuantum`** stays as a debug knob,
   documented as null-result.

## What to defer

1. Approach γ (broader scheduling-trace replay) — defer until a
   future cap demonstrably scheduling-related appears.
2. Approach β / α (deterministic preemption / cooperative canary) —
   defer indefinitely.

## What to do next

The next phase is **C+24** (or whatever the natural next number) on
the head divergence at idx 105,046: `VdInitializeEngines.return_value`
(canary=1 ours=0). This is a regular engine bug investigation, ~5-50
LOC.

## Fallback: γ trigger criteria

If a future phase finds a NEW scheduling-determinism cap (defined as:
two consecutive divergences whose root cause is contention/wakeup-
ordering across ≥2 guest threads, NOT a guest-code bug or kernel
emit-completeness gap), then revisit γ. The criteria:

- The new cap is ≥1,000 events long.
- The C+21 / D-extension absorbers cannot fold it within their
  current cap (32 pairs).
- Empirical jitter sampling (≥3 canary colds) confirms structural
  shape divergence, not just SID identity drift.

If all three hold, γ is justified. Estimated ~600 LOC across 4-5
sessions.

## What this recommendation is NOT

- It is NOT "no scheduling work was useful." Stages 1-4 + D-extension
  produced the matched-prefix advance from 104,606 → 105,046 (+440).
- It is NOT "the absorbers are perfect forever." They are explicit
  band-aids in spirit of reading-error #23, annotated in schema-v1.md
  v1.5.
- It is NOT "ours and canary are bit-aligned in contention regions."
  They are *measurably* aligned (matched-prefix) but not *structurally*
  aligned (the underlying guest events still differ; the absorber
  folds the difference).

## Multi-session budget if we proceed (γ scenario only)

Sessions estimated 4-5. NOT scheduled now.

| stage | LOC | est session |
|---|---|---|
| γ-Stage 1: extend canary trace to wake/park/yield | ~150 | 1 |
| γ-Stage 2: extend manifest builder | ~80 | 0.5 |
| γ-Stage 3: generalized replayer in ours | ~250 | 2 |
| γ-Stage 4: diff-tool integration | ~50 | 0.5 |
| γ-Stage 5: validation + sister budgets | n/a | 1 |
| **total** | **~530** | **~5** |

## Acceptance for THIS session (planning-only)

- [x] Planning artifacts in `audit-runs/phase-c23-scheduler-determinism-plan/`.
- [x] Engine sources UNCHANGED (verified by file listing — only
  documentation + 1 python probe written).
- [x] Diff tool UNCHANGED.
- [x] Memory entry to be written next.
- [x] Recommendation justified against C+21 band-aid + breadth of
  contention regions + multi-session budget.