# Phase C+21 cold-vs-cold result (2026-05-14)

## Matched-prefix table (vs C+19/C+20 baseline)

| chain                          | C+19    | C+20    | C+21 (jitter-1) | C+21 (jitter-2) | C+21 (jitter-3) | C+21 (fresh c21) |
|--------------------------------|---------|---------|-----------------|-----------------|-----------------|------------------|
| canary tid=6 → ours tid=1 main | 104,606 | 104,606 | **104,607**     | **104,607**     | **104,607**     | **104,607**      |
| canary tid=4 → ours tid=11     | 11      | 11      | 11              | 11              | 11              | 11               |
| canary tid=7 → ours tid=2      | 32      | 32      | 32              | 32              | 32              | 32               |
| canary tid=12 → ours tid=7     | 3       | 3       | 3               | 3               | 3               | 3                |
| canary tid=14 → ours tid=9     | 41      | 41      | 41              | 41              | 41              | 41               |
| canary tid=15 → ours tid=10    | 16      | 16      | 16              | 16              | 16              | 16               |

Main chain ADVANCED +1 vs C+19/C+20 baseline (104,606 → 104,607).
The +1 is unmasking the next real downstream divergence; the C+20
wait.begin jitter event at 104,606 is now transparent.

CRITICALLY: all 4 canary cold samples now agree at 104,607 —
reading-error #32 jitter cloud is fully absorbed at the diff layer.

## Floating-event absorption counts

| run                | floating_create (c/o) | floating_wait (c/o) |
|--------------------|-----------------------|---------------------|
| jitter-1           | 0 / 1 (tid=15→10)    | 1 / 0 (tid=6→1)     |
| jitter-2           | 0 / 1                 | 0 / 0               |
| jitter-3           | 1 / 0 + 0 / 1         | 3 / 0 (tid=6→1)     |
| fresh c21          | 0 / 1                 | 0 / 0               |

`floating_create` reflects C+18 absorption — observed
in tid=15→10 (the original D-NEW-3 case, still firing 1 ours-side
handle.create absorb). `floating_wait` is new in C+21 and varies
with canary's host-scheduler contention pattern. Sister chains see
no wait.begin absorption (their waits are per-thread, not shared-
global — strict matching preserved).

## Cold-stable invariants

- **ours cold digest**: bit-stable across 3 runs of `xrs-verify-c19`
  (50M instr, 39290 imports, 0 unimpl). No engine source change.
- **Phase B image hash**: unchanged (no engine source change).
- **Schema version**: still `1` (wire format unchanged — C+21 is
  diff-tool-only).

## Reading-error class #32 — diff-tool counter-measure landed

Pre-C+21, matched-prefix in canary cold-vs-cold could vary by 1+
indices across canary samples (jitter-1 contended → matched=104,606
strict-fail; jitter-2 fast-path → matched=104,607 naturally;
jitter-3 shifted → matched=104,603 strict-fail). Post-C+21, all
samples yield matched=104,607 — the metric is now stable across
canary's contention jitter.

## Outcome

C+21 LANDED. Main matched-prefix 104,606 → 104,607 (+1) and
deterministic across canary cold samples. Reading-error class #32
mitigated at the diff-tool layer. The next real divergence at
104,607 is an actual structural difference (canary enters another
CS while ours leaves one) — handed off to C+22.

## Next target

C+22 = the new divergence at tid=6→1 idx=104,607 (canary
`import.call RtlEnterCriticalSection` vs ours
`import.call RtlLeaveCriticalSection`). Initial framing: appears to
be a CS-acquire-vs-release ordering difference in the post-fast-
path RtlEnterCriticalSection sequence. Investigation will start
with a static read of canary's vs ours's RtlEnter and RtlLeave
control flow at this guest PC range.

Alternative: tackle D-NEW-2 (`KeWaitForSingleObject` `timeout_ns`
sign/scale asymmetry on tid=12→7 idx=3) — small ε-class encoding
fix, independent of scheduler determinism. Recommended path
depends on cost estimate; the C+22 framing scope can pick either.