Guard the parity pin header against declaration-order breaks (#558) - #594

…header main already fixed (#558)

FOLLOWING_AGENTS_PROTOCOL

`main` fixed the declaration-order break itself at `fafa16f0f` (PR #556) while
this branch carried its own repair, and #551/#546 are CLOSED. Two independent
moves of one block do not conflict: merging the two repairs auto-merged clean
into a header that DEFINED `Nemotron35LightningSnapshot()` twice and re-broke
the same 14 TUs. So this branch was RESET onto `fafa16f0f` and re-applies only
the part that is still owed -- the guard, now tracked by #558.
`git diff origin/main -- tests/parity/hf_snapshot.h` is EMPTY: main's header
survives byte-for-byte (md5 048c84eb5cc20146f4be45f7747b8a34).

WHY THE GUARD IS STILL WANTED. Nothing else in the Python lane sees the defect
-- delete this method and `scripts/check-snapshot-pins.py` exits 0 on a header
no TU can include. A full C++ build does catch it, and that is exactly the
point: the commit that broke it was records and evidence and never built C++ at
all, and all 14 TUs that include the header are checkpoint-gated, so a serial
`ctest` reported `***Not Run`, which on a box with no snapshots reads as a
missing checkpoint rather than a build break. This suite already runs from
`scripts/agent-preflight.sh` and from CI's record lane (`ci.yml:172`), needs no
CMake, no build tree, no GPU and no checkpoint, and costs ~0.3s.

TWO REVIEW FINDINGS CLOSED WITH IT.

1. A SKIP WAS INVISIBLE. `skipTest` exits 0 and `agent-preflight.sh`'s `run()`
   prints a green `ok` while swallowing stdout, so a compiler-less box turned
   the guard into a silent no-op. Measured before:
   `env -u CXX CI=1 PATH=/nonexistent python3 -m unittest ...` -> `OK
   (skipped=1)`, exit 0. It now FAILS when `CI` is set and skips otherwise:
   with `CI=1` and no compiler, `AssertionError: CI must be able to
   syntax-check tests/parity/hf_snapshot.h ... A skip here exits 0 and reads as
   a pass`, exit 1; without `CI`, `OK (skipped=1)`, exit 0.

2. `$CXX` LOST ITS LAUNCHER. `configured[:1]` kept only the first token, so
   `ccache g++` / `sccache clang++` / `distcc g++` ran the LAUNCHER with the
   compiler's flags. Measured before, with `CXX="env g++"`:
   `AssertionError: 0 != 125 ... /usr/bin/env: invalid option -- 's'` -- a red
   naming this header for a defect in the environment. The whole token list is
   now the argv prefix; the same `CXX="env g++"` is green. A candidate that
   cannot be LAUNCHED (126/127) falls through to the next; every other status
   is a compiler's verdict and is reported, so `CXX` exiting 1 is still RED and
   is not masked by the fallback.

NEGATIVE MUTATION, against main's fixed header, in a scratch tree (the repo
copy was never touched -- md5 unchanged before and after):
  * M1, the #551 ordering restored: FAILED, `'HfSnapshot' was not declared in
    this scope`, quoted verbatim by the assertion message.
  * M2, the auto-merge hazard above (the block defined twice): FAILED,
    `redefinition of 'std::string parity::Nemotron35LightningSnapshot()'`.
    No other check in the repo sees that one.

The intake row is #558 and not #551: the table is for OPEN issues, #551 closed
at 2026-08-12T23:53:23Z, and the row `f2fe496f` added went with the reset.
`Row` is `—` -- the guard owns no capability, so no matrix row and no count
that `check-agent-record.py` enforces changes. Placement is sorted, not
appended: within the `—` entries the file orders by issue number ascending and
558 is the highest, so its sort position is the last row. The anchor was
asserted unique (count == 1) before the insert; the result is 1 insertion, 0
deletions.

Gates on the final tree: clean out-of-tree `-Werror` Release build on local
disk (`build-gate/`, CPU: no CUDA toolkit on this box) exit 0; SERIAL `ctest`
exit 0; `scripts/agent-preflight.sh` exit 0 before the edits and `--staged`
before this commit. The 13 checkpoint-gated parity suites emit their loud
checkpoint-absent SKIP with 0 assertions and are NOT presented as coverage; no
GPU gate and no checkpoint gate exists on this box.

Following-Agents-Protocol: true
AI-Assisted: true
Assisted-by: AGENT:claude-opus-5 [Claude Code]