docs: U9 --- the first red in this family with an in-run control

The merge gate for this lane failed at `11-sweep` on two selectors, and
both had already passed in `03-lib` and `04-lib-crdt` of the SAME gate
invocation, minutes earlier, on the same tree and machine. U6 and U7
could only ever compare a red run against a different run; this is the
first occurrence in the family where the control is inside the run, and
that is what the row is for.

Both are near misses against existing rows, and neither is folded in:

- The PTY failure carries U2's exact fragment, but U2's selector field
  names only the *raw* selector. U2's occurrence 2 had raw and canonical
  failing together; here canonical redded ALONE and raw passed, which
  U2's evidence has never shown.
- `composition_overhead_under_ten_percent` is one of U6's two selectors,
  and U6 instructs in its own text that one-without-the-other is a
  different incident. It redded without its pair, in a different step,
  at 1.613x against U6's 1.297x. Judged as instructed.

The row also records the first checkable candidate this family has had.
`cargo test --workspace` runs many test binaries concurrently while
`--lib` runs one, so the passing and failing steps differ in kind and
not merely in load average --- with a stated control that separates
load from concurrency. U6 and U7 both left the confound atmospheric
and unmeasured; this does not measure it either, but it names something
that can be.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016bqGA6s9tTUFzYpbeW3tai

docs/ci-red-signatures.md

@@ -773,3 +773,29 @@ very mistake U3 is named for.
 | **what is NOT** | everything else. Without the assertion text this cannot be matched against a future occurrence, which is the whole purpose of a row here |
 | **why it matters anyway** | it is the **third distinct macOS selector** to red in one session, after U4 (`full_grid_resync`) and U5 (`ctrl_c_during_reconnect`). Three unrelated selectors failing on the macOS legs suggests a **background failure rate on that platform** rather than three independent test bugs — and that materially affects any equal-rate reasoning about which branch a failure "landed on" |
 | **next occurrence** | **read the log BEFORE rerunning anything.** That is U3's stated lesson and this row is its fourth violation |
+
+### U9 — a PTY test and a budget test red **together** in one `11-sweep`, with an in-run control
+
+Recorded on the `destination-capture` merge tree, 2026-08-10, in the
+gate run that was meant to clear PR #231.
+
+**This row's value is its control, not its selectors.** U6 and U7 could
+only compare a red run against a *different* run. Here both selectors
+ran green **inside the same gate invocation**, minutes earlier, on the
+same tree and machine — `03-lib` (1928 passed, 0 failed) and
+`04-lib-crdt` (2113 passed, 0 failed) — and then failed in `11-sweep`.
+Whatever this is, it is not the tree.
+
+| field | value |
+|---|---|
+| **selector** | `--lib process::tests::m6_1_pty_canonical_mode_keeps_kernel_echo` **and** `editor::tests::composition_overhead_under_ten_percent`, failing in the same `11-sweep` step |
+| **job / flavor** | local (Linux), `scripts/gate` step `11-sweep` (`cargo test --workspace --no-fail-fast -- --skip basedpyright`), fresh per-lane target dir, no sibling worktrees building |
+| **required fragments** | ``canonical mode should leave echo enabled (no `-echo` flag); stty -a output was: ""`` **and** `composition machinery added more than 10% overhead` |
+| **NOT fragments** | the measured numbers (`1.613`, `single=191935 ns`, `dispatch=309602 ns`) and every `:LINE` suffix — occurrence-specific |
+| **status** | **one occurrence; INTERMITTENT — the identical sweep command on the same tree was green (118 targets, 1928 passed, exit 0)** |
+| **what IS established** | intermittence, with the strongest available exclusion of the tree: green in two earlier steps of the **same run**, green isolated afterwards (`2 passed`, 1.70 s), green on a full sweep rerun. Both assertions are **timing-sensitive by construction** — one reads collected child output within a deadline, the other measures wall-clock composition overhead (observed 1.613× against a 1.10× budget; 61.3% dispatch and 124.6% realistic overhead) |
+| **what is NOT** | cause, and the load confound is **partially measured but NOT controlled**. The failing sweep ran inside a full gate; the green rerun started at load average 1.98 with the 5-minute figure still at 8.03 from that gate. Different conditions is not a measurement of the mechanism, and this row does not treat it as one |
+| **the structural difference worth testing next** | `cargo test --workspace` runs **many test binaries concurrently**; `--lib` runs **one**. That is a difference in kind between the passing steps and the failing one, not merely a difference in load average — and it is the first candidate this family has had that is checkable rather than atmospheric. **Discriminating control:** rerun the sweep with test-binary concurrency pinned to 1, and separately run the `--lib` binary alone under synthetic load. A red under synthetic load at low sweep concurrency implicates load; a red at high concurrency and low load implicates the concurrency itself |
+| **relation to U2 — a NEAR MISS, do not match it there** | the PTY fragment is U2's exact family (`stty -a output was: ""`), but U2's selector field names only `m6_1_pty_raw_mode_disables_kernel_echo`. U2's occurrence 2 saw raw **and** canonical fail together; here **canonical redded alone and raw passed**, which U2's evidence has never shown. It is recorded here rather than folded into U2 so that the "canonical alone" case stays visible |
+| **relation to U6 — its own instruction, honoured** | `composition_overhead_under_ten_percent` is one of U6's two selectors, and U6 says plainly: "If a future run reds **one** of these without the other, that is a different incident and should be judged as one." It redded without `criterion_1_end_of_line_typing…`, in a different step, at a far larger margin (1.613× here against U6's 1.297×). Judged as a different incident, as instructed |
+| **what this row does NOT assert** | that the two selectors share a mechanism. They failed together once; they belong to different subsystems; and U7 already refused this exact merge for U6. The **co-failure inside one step with an in-run green control** is the signature — not either name, and not a shared cause |