fix(pcie-probe): select a card by ordinal only, honour probe failures, refuse blind

Per-card measurement was unreachable for every card but ordinal 0: the driver
set HIP_VISIBLE_DEVICES=N and still passed N as the device argument, so the
probe called hipSetDevice(N) on a runtime where the mask left only ordinal 0.
The bats stub reported the env var rather than the argument it received, so
the suite asserted the mask was set and never noticed the collision.

- one selector: the ordinal is passed as argv, the mask is never set
- a probe that fails is now exit 5 and names probe + device; the sweep still
  measures the remaining cards instead of swallowing the failure
- the guard fails closed when `fuser` is missing (gpu_busy_percent reads 0 for
  a resident-but-idle service), with PCIE_PROBE_FUSER as the test seam
- stub probes now model the HIP mask rule; the pre-existing per-device test
  goes red against the old driver, which is the regression proof

Refs #60

omp-docs/tools/pcie-probe.md

@@ -8,7 +8,7 @@ tags:
   - rdna4
   - benchmarking
   - dankserver
-verified: 2026-10-20
+verified: 2026-10-21
 timestamp: 2026-10-20
 ---
 
@@ -40,7 +40,10 @@ scripts/pcie-probe.sh --build-only         # compile, never touch the cards
 
 The guard runs **before anything is built or run** and refuses when the GPU is
 occupied — `gpu_busy_percent` above `PCIE_PROBE_MAX_BUSY` (default 5 %) or any
-process holding `/dev/kfd`. Refusal is exit `3`, and it stops nothing.
+process holding `/dev/kfd`. Refusal is exit `3`, and it stops nothing. If the holder
+probe is missing the guard refuses too rather than assuming idle — see
+[seams](#exit-status-and-seams); `gpu_busy_percent` reads `0` for a service that is
+resident and idle, so a silent fallback is exactly the benchmark this law forbids.
 
 Auto-stopping a production endpoint is not this tool's decision to make, and a
 measurement taken as noise beside serving traffic is worthless anyway: it is
@@ -65,8 +68,11 @@ throughput at *that* size is what matters, not the big-transfer figure:
 - Pinned versus pageable columns apart means pinning is load-bearing; together
   means the copies are already pinned by someone else.
 
-Measure one device at a time (`--device`). A run that lets the runtime pick
-several devices cannot attribute bandwidth to a card at all.
+Measure one device at a time (`--device`, a HIP ordinal). A run that lets the runtime
+pick several devices cannot attribute bandwidth to a card at all. The ordinal goes to
+the probe as an argument and **the driver never sets `HIP_VISIBLE_DEVICES`**: masking
+the device *and* passing its real ordinal asks `hipSetDevice` for a card the runtime
+has hidden, so every card but ordinal 0 fails to measure at all. One selector only.
 
 ## The trap carried in the sources
 
@@ -79,14 +85,18 @@ when touching these files.
 
 ## Exit status and seams
 
-`0` measured · `2` usage or no sources · `3` refused, GPU busy · `4` no HIP
-toolchain or build failed. A client box without the SDK exits `4` loudly rather
-than reporting an empty success; the probes belong on the box with the cards.
+`0` measured · `2` usage or no sources · `3` refused, GPU busy or holders unknowable ·
+`4` no HIP toolchain or build failed · `5` a probe ran and failed. A client box without
+the SDK exits `4` loudly rather than reporting an empty success; the probes belong on
+the box with the cards. A probe that dies (the segfault above, an unfundable
+allocation) exits `5` and names which probe on which device — a sweep that quietly
+swallowed the failure would report success with half its rows missing.
 
 Test seams (all documented in the script header): `PCIE_PROBE_SRC`,
 `PCIE_PROBE_BUILD`, `PCIE_PROBE_SYSFS`, `PCIE_PROBE_KFD`, `PCIE_PROBE_HIPCC`,
-`PCIE_PROBE_MAX_BUSY` — the sysfs/kfd paths are what make a refusal testable on
-a machine with no cards, which is the only honest way to test a refusal.
+`PCIE_PROBE_FUSER`, `PCIE_PROBE_MAX_BUSY` — the sysfs/kfd paths are what make a
+refusal testable on a machine with no cards, which is the only honest way to test a
+refusal.
 
 Measured campaign numbers are not kept here; they belong with the endpoint they
 were taken on, in the serverAppConfigs LLM runbook vault.