volt: write-axis -- guard manifest_audit.py's --json write, probe it; leg green 8/8

manifest_audit.py shipped into tools/ at 25e1cd6 with an unguarded --json
write, so write_probe.py graded it NO_PROBE and verify.py's write-axis leg
read FAULT (rc 1) on every run.

 - tools/manifest_audit.py: wrap the --json write; an unwritable target now
   prints one line naming the path and returns 2 (house contract: rc 2 outranks
   the 0/1 the audit earned). Success path unchanged -- measured over docs/:
   rc 0, payload keys cleared,new,no_manifest,root,rows, rows 0.
 - tools/write_probe.py: WRITE_PROBES row (--root {tinyrepo} --json {target}).

Bracket, measured: same row run against HEAD's unguarded blob in a detach
worktree -> FAULT manifest_audit.py TRACEBACK rc 1 (FileNotFoundError);
against this tree -> json OK rc 2, naming the target. write_probe --selftest
15/15, manifest_audit --selftest 17/17, verify.py --repo . 8/8 legs pass,
selftest-sweep pin 22 pinned / 22 agree.

tools/manifest_audit.py

@@ -46,8 +46,12 @@ WHY THIS IS IN `tools/` AND IN NO verify.py LEG
   report a machine-local path as a repo verdict -- the "advice that names a
   target which does not exist" defect `verify.py` opens with, one level down.
   It is discovered by `selftest_sweep.py` from `tools/*.py` and graded by both
-  of that sweep's input axes and by the count pin (17 cells), which is what a
-  tool in no leg gets: a tool in no leg is ungraded nowhere.
+  of that sweep's input axes and by the count pin (17 cells). The write end is
+  graded too, by `write_probe.py`'s json axis -- that probe runs this tool
+  against a throwaway fixture root, so the fixture, not the host, is what the
+  verdict describes. What no leg does is grade `--root` against this repository:
+  that stays a caller's explicit request, because the artifact tree a reader
+  cares about is theirs, not the checkout's.
 
   The two axes it is graded on, and why the third does not apply to it:
     read-refusal    --root that does not exist, and --root naming a file, are
@@ -401,9 +405,18 @@ def main(argv=None) -> int:
             "new": [{"dir": r["dir"], "class": r["verdict"]} for r in new],
             "cleared": [{"dir": d, "class": c} for d, c in cleared],
         }
+        try:
             with open(args.json, "w", encoding="utf-8") as fh:
                 json.dump(blob, fh, indent=2, sort_keys=True)
                 fh.write("\n")
+        except OSError as exc:
+            # House contract, graded by tools/write_probe.py: a target this host
+            # cannot write is rc 2 with one line naming it. rc 2 outranks the
+            # 0/1 the audit earned -- a caller who asked for a file and got a
+            # traceback cannot know a grade happened at all.
+            print("manifest_audit: cannot write --json %s: %s" % (args.json, exc),
+                  file=sys.stderr)
+            return 2
     if (args.fail or args.strict) and new:
         return 1
     return 0

tools/write_probe.py

@@ -168,6 +168,7 @@ WRITE_PROBES = (
     ("gate_blame.py", ["--repo", "{tinyrepo}", "--leg", "selftest-sweep",
                        "--range", "1", "--json", "{target}"]),
     ("landing_check.py", ["--repo", "{tinyrepo}", "--json", "{target}"]),
+    ("manifest_audit.py", ["--root", "{tinyrepo}", "--json", "{target}"]),
     ("path_claim_check.py", ["--repo", "{tinyrepo}", "--json", "{target}"]),
     ("queue_append_audit.py", ["--repo", "{tinyrepo}", "--json", "{target}"]),
     ("queue_claim_check.py", ["--queue", "{queue}", "--json", "{target}"]),