ci(fpga): pipefail the conformance step so a failing runner fails the job

.github/workflows/fpga-build.yml

@@ -760,6 +760,12 @@ jobs:
           # is the executed-vector registry in tools/run_conformance_vvp.py
           # (mac today); every module and group outside it is printed as debt,
           # never silently counted as covered.
+          # #2415: without pipefail the `if` below tests tee's exit status, not
+          # the runner's -- so a MISMATCH printed one line above was reported as
+          # "EXECUTED, all cases passed" and the job stayed green. Same defect
+          # #2242 removed from fpga-formal; the corrected pattern is ~115 lines
+          # above in this file.
+          set -o pipefail
           echo "## Conformance Vector Execution" >> $GITHUB_STEP_SUMMARY
           echo "" >> $GITHUB_STEP_SUMMARY
           mkdir -p build/fpga/conformance

docs/now/2026-08-22-conformance-pipefail.md

@@ -0,0 +1,58 @@
+# NOW — the gate said "all cases passed" one line below its own MISMATCH
+
+Last updated: 2026-08-22
+
+## Add pipefail to the conformance execution step (Closes #2415)
+
+- Branch: `fix/2415-conformance-pipefail`
+- Issue: #2415 · lands on top of #2403
+
+### Что легло
+
+One line in `.github/workflows/fpga-build.yml` — `set -o pipefail` at the top of the
+"Execute conformance vectors" step — plus the comment explaining why.
+
+`if python3 tools/run_conformance_vvp.py ... | tee ...; then` tests **tee's** exit status
+without it. A failing runner took the `then` branch, `fails` stayed 0, and the `exit 1`
+below never fired. The step has no `shell: bash` and the workflow has no `defaults:` block,
+so the default `bash -e {0}` applies and `-o pipefail` is not on.
+
+**This is the defect #2242 removed from `fpga-formal`**, in the same file, 115 lines below
+the corrected pattern at `:638-640`.
+
+### Границы честности (BINDING)
+
+- **The rest of #2403 is untouched and is good.** Registry-scoped execution with the
+  remainder printed as visible debt is the honest shape. This changes one line.
+- **This does not establish that the vectors pass.** It establishes that if they stop
+  passing, the job will say so. Whether `fpga-conformance` is green for the right reason is
+  now measurable; before it was not.
+- **The planted-fault control in #2403 validated the runner, not the gate.** Different
+  claims. The control should be re-run *through the workflow step* so it covers the gate;
+  not done here.
+- Raised on #2403 before it merged and merged unchanged. Context, not complaint — the
+  review landed close to the merge.
+
+### Evidence
+
+Stub runner failing the way a real mismatch would:
+
+```
+echo "case 3: MISMATCH expected 0x2a got 0x00"; exit 1
+```
+
+As landed on master:
+
+```
+case 3: MISMATCH expected 0x2a got 0x00
+| mac | EXECUTED, all cases passed |
+fails=0                                    <- step exit 0, job green
+```
+
+With `set -o pipefail`, same runner:
+
+```
+| mac | FAILED |
+fails=1
+::error::conformance execution failed      <- step exit 1
+```