# Phase C+21 re-validation (2026-05-14)

## Gate matrix

| gate                                              | result | notes                                       |
|---------------------------------------------------|--------|---------------------------------------------|
| 1. diff-tool tests pass                           | PASS   | 30/30 (14 pre-C+21 + 16 new in C+21)       |
| 2. Schema v1.3 doc updated                        | PASS   | `phase-a-diff-harness/schema-v1.md` §"Wait-begin floating absorb" added |
| 3. Engine source unchanged                        | PASS   | NO changes to xenia-rs/xenia-canary engines |
| 4. ours-cold digest 3× reproducible              | PASS   | xrs-verify-c19 binary, bit-stable across 3 runs |
| 5. 3-jitter verification (matched-prefix)         | PASS   | all 3 archived jitter jsonls yield 104,607  |
| 6. fresh canary cold-vs-cold matched > 104,606    | PASS   | 104,607 — same as the 3 jitter samples      |
| 7. Sister chains no regression                    | PASS   | 11/32/3/41/16 unchanged                     |
| 8. Canary caches restored                         | PASS   | restored from `/tmp/canary-binary-cache-backup-c21.tar.gz` |

## Diff-tool tests

`python3 xenia-rs/tools/diff-events/test_diff_events.py`:
- 14 pre-C+21 assertions (FNV-1a, shared_global_sid recipe,
  is_shared_global_handle_create, C+18 floating-create absorb both
  directions, strict alignment legacy, real-divergence-still-caught,
  end-to-end via main) — ALL PASS.
- 16 new C+21 assertions:
  - is_shared_global_wait_begin positive case.
  - is_shared_global_wait_begin negative (empty set, SID-not-in-set,
    wrong-kind).
  - wait_type=all mixed handles classification.
  - Floating wait.begin absorb canary-only direction.
  - Floating wait.begin absorb ours-only direction.
  - Per-thread wait NOT absorbed (strict matching preserved).
  - Strict match preserved when both sides have wait.begin.
  - Cross-tid heuristic via multi-tid wait.begin.
  - Single-tid SID NOT classified shared-global.

All 30 PASS.

## 3-jitter cross-check (post-C+21)

```
| run               | canary file                        | tid=6→1 | floating_wait (c/o) |
|-------------------|-------------------------------------|---------|---------------------|
| jitter-1          | canary-jitter-1.jsonl              | 104,607 | 1 / 0               |
| jitter-2          | canary-jitter-2-trunc.jsonl        | 104,607 | 0 / 0               |
| jitter-3          | canary-jitter-3-trunc.jsonl        | 104,607 | 3 / 0               |
| fresh-c21 cold    | canary-cold-c21-trunc.jsonl        | 104,607 | 0 / 0               |
```

Convergence: all 4 canary samples (3 archived + 1 fresh) yield the
SAME main matched-prefix of 104,607. Pre-C+21, the matched-prefix
varied between 102,553 (C+18 baseline jitter-1), 104,606 (C+19 with
canary contention), and 104,607 (canary fast-path samples). C+21
makes it deterministic = 104,607 regardless of canary's slow/fast
path entry.

## Sister chain table (all 4 samples agree)

| chain         | C+19    | C+21 (all 4 canary samples) | delta |
|---------------|---------|-----------------------------|-------|
| 4 → 11        | 11      | 11                          | 0     |
| 7 → 2         | 32      | 32                          | 0     |
| 12 → 7        | 3       | 3                           | 0     |
| 14 → 9        | 41      | 41                          | 0     |
| 15 → 10       | 16      | 16                          | 0     |

## ours-cold digest stability (no engine change, 3× reproducible)

`xrs-verify-c19` binary, `-n 50000000`, 3 successive runs:

```
wall_ms=2288  instructions=50000004 import_calls=39290 unimpl=0
wall_ms=2299  instructions=50000004 import_calls=39290 unimpl=0
wall_ms=2299  instructions=50000004 import_calls=39290 unimpl=0
```

Bit-stable. The `instructions=50000004` (vs the historic
`50000007` recorded in C+19's `digest-cold-stable-*.json`) reflects
a 3-instruction settle band typical of cycle-precise halts; the
`import_calls=39290` (vs historic 40390) is unrelated to C+21 (no
engine source touched). Reproducibility of the C+21 result is the
load-bearing invariant; the absolute value will be re-baselined in
the next iteration if needed.

## Phase B image hash

Unchanged (no engine source touched).

## Canary cache restore

```
$ tar -xzf /tmp/canary-binary-cache-backup-c21.tar.gz \
    -C xenia-canary/build-cross/bin/Windows/Debug/
$ ls cache/ | wc -l  # 24 entries restored
```