feat(frontmatter): --frontmatter KEY=VALUE stamps a key on every concept of a run

K3-19 b. `okf build --frontmatter KEY=VALUE`, repeatable, split on the FIRST
'=' (`cli.frontmatter_from_flags`) because a publisher's address carries '='
itself. The value is written verbatim on ONE line: the block form of a
`sources` list is invisible to this package's line-oriented readers, so the
flow form is the only one that survives them. Also reachable as
`build(frontmatter=...)`, `measure(concept_frontmatter_values=...)`,
`process_inbox(concept_frontmatter_values=...)` and
`render_inbox_concept(concept_frontmatter_values=...)`, keyword-only with
defaults, so every existing call site is source-compatible.

Precedence: a stated value beats what the document declares, which beats the
file name. A run may ADD any key and REPLACE only `sources` and
`description` -- the two with a derived layer below the flag. Every other key
the door writes is refused by `inbox.validate_concept_frontmatter` before a
proposal is written (`run_frontmatter_invalid`): measured from the bytes, owned
by another argument, the ownership stamp a later run reads back (including
Door A's `ingest_manifest`, which would make that door claim a Door B file),
or a derived facet whose `derived` marker would go on naming a replaced value.
A value that would not read back as stated -- empty, multi-line, or padded,
since `parse_frontmatter` strips -- is refused too.

SPEC SS 4.1 "Extensions" lets a producer add any key and SS 11 forbids a
consumer to reject one. Without the flag nothing moves: a test holds the
flagged tree to the plain one minus exactly the stated line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/llm_ingestion_okf/corpus.py

@@ -283,6 +283,7 @@ def measure(
     pdf_headings: bool = False,
     heading_reserve: Callable[[str], bool] | None = None,
     ocr: bool = False,
+    concept_frontmatter_values: Mapping[str, str] | None = None,
 ) -> CorpusReport:
     """Run the corpus through the door and count what happened.
 
@@ -307,6 +308,7 @@ def measure(
         pdf_headings=pdf_headings,
         heading_reserve=heading_reserve,
         ocr=ocr,
+        concept_frontmatter_values=concept_frontmatter_values,
     )
     elapsed = time.monotonic() - started