feat(validation): report an unexpected exception as a validator fault

docs/architecture.md

@@ -151,6 +151,14 @@ in the vendored catalogue, never from a hardcoded column in the check itself. Th
 one code base validate against several specification versions at once: relaxing a `MUST` to a
 `SHOULD` upstream turns a failure into a warning here with no code change.
 
+One severity does not come from the catalogue, because it is not about the document at all. A
+check that raises something it does not expect reports `fault`: a defect in this package, not a
+finding. It fails the run like `fail` does, but says the check produced no verdict, so the
+document is neither condemned nor cleared. A fault keeps the id of the check that raised it
+rather than reporting under one of its own, so "which part of the validator broke" is answered by
+the same identifier that names what it was trying to establish, and the run still reports every
+other target - one broken check costs one check.
+
 A rule absent from the selected version's catalogue, or marked deprecated there, is skipped with
 a message saying so, rather than checked anyway - older meta-schema versions ship no catalogue at
 all and skip the whole `rule.*` family. A false positive costs far more than a missed finding,

src/oold/validation/cli.py

@@ -21,7 +21,7 @@
 from .meta_store import MetaSchemaError, describe_store, fetch_remote, load_index, resolve_selection
 from .meta_vendor import vendor_version
 from .pipeline import Options, run_compliance, validate_directory, validate_instance, validate_schema
-from .report import FAIL, OK, SKIP, WARN, Report
+from .report import FAIL, FAULT, OK, SKIP, WARN, Report
 
 EXIT_OK = 0
 EXIT_FAILED = 1
@@ -86,6 +86,9 @@ def _classify(target: Path) -> str:
     FAIL: {"fg": "red", "bold": True},
     WARN: {"fg": "yellow"},
     SKIP: {"fg": "cyan"},
+    # Brighter than FAIL on purpose: a fault is our bug, and the reader should not spend time
+    # looking for it in their own document.
+    FAULT: {"fg": "magenta", "bold": True},
 }
 
 _meta_option = click.option(
@@ -136,17 +139,20 @@ def _print_human(report: Report, verbose: bool) -> None:
     versions = ", ".join(report.meta_versions) or "none"
     click.echo(f"{status}  {report.source}")
     click.echo(f"      meta-schema: {versions}")
+    # Faults appear only when there are some. A permanent "0 fault(s)" would train the reader to
+    # skip the field, which is the opposite of what it is for.
+    faults = f", {counts[FAULT]} VALIDATOR FAULT(S)" if counts[FAULT] else ""
     click.echo(
         f"      {counts[OK]} ok, {counts[FAIL]} failed, {counts[WARN]} warning(s), "
-        f"{counts[SKIP]} skipped, across {len(report.targets())} target(s)"
+        f"{counts[SKIP]} skipped{faults}, across {len(report.targets())} target(s)"
     )
 
     shown = report.checks if verbose else [c for c in report.checks if c.status != OK]
     if shown:
         click.echo()
         for check in shown:
             style = _STATUS_STYLE.get(check.status, {})
-            label = click.style(check.status.upper().ljust(4), **style)
+            label = click.style(check.status.upper().ljust(5), **style)
             rule = click.style(f" {check.rule}", fg="blue") if check.rule else ""
             version = f" [{check.meta_version}]" if check.meta_version else ""
             message = f": {check.message}" if check.message else ""

src/oold/validation/mcp_server.py

@@ -60,7 +60,11 @@ class CheckResult(BaseModel):
 
     id: str = Field(description="The check id, e.g. lint.container or rule.id-fragment.")
     target: str = Field(description="What was checked: a schema file, an instance, or a directory entry.")
-    status: str = Field(description="ok, fail, warn, or skip.")
+    status: str = Field(
+        description="ok, fail, warn, skip, or fault. A fault is a defect in the validator: the "
+        "check raised something it does not expect, so it produced no verdict and the document "
+        "is neither condemned nor cleared. Like fail it makes the run not pass."
+    )
     message: str = Field(default="", description="Why the check produced this status, when it is not ok.")
     detail: dict[str, Any] | None = Field(
         default=None, description="Extra structured detail behind the message. Only present with verbosity='full'."