fix(okf): the index entry point never requires 'type', frontmatter or not

navigate_bundle died on an index.md that carried a frontmatter block without
a 'type' field: _parse_index_entry keyed its tolerance on the ABSENCE of the
block, so a frontmatter-ful index fell through to parse_concept_file and
raised. Not merely the index read failed — the whole navigation did.

OKF v0.2 (A-E6) triggers this: it stamps okf_version into index.md. Our own
assumption ("a generated index has no frontmatter") is frozen in the golden,
so nothing caught it. The defect is ours, not theirs.

method-spec §3 Step 1 renders the index as "the index body (the summary)"
and every OTHER file as a "non-index concept file" — the index is not a
concept file, so the 'type' requirement never reached it in the first place.
The tolerance is now keyed on being the index, which is what it always meant.

Load-bearing, both directions detach-proved:
- test_index_with_frontmatter_lacking_type_navigates goes RED when the
  tolerance is re-keyed on absence-of-frontmatter.
- test_missing_type_is_an_error goes RED when the index default leaks onto
  concept files. Its vehicle moved from index.md to a non-index file: it
  proves the concept-file rule, and an index.md vehicle now proves the
  opposite of what the test is named for. This also closes a real gap —
  nothing tested a non-index file WITH frontmatter but WITHOUT 'type'.

Goldens untouched (shared bundle has type: index; ingest golden index has no
frontmatter). Suite 627 -> 628 passed.

src/portfolio_optimiser_claude/okf.py

@@ -47,10 +47,11 @@ def _strip_matching_quotes(value: str) -> str:
     return value
 
 
-def parse_concept_file(path: Path) -> ConceptFile:
-    """Parse frontmatter (leading ``---`` block, line-oriented ``key: value``) + body.
+def _parse_frontmatter_and_body(path: Path) -> tuple[dict[str, str], str]:
+    """Split a leading ``---`` block (line-oriented ``key: value``) from the body.
 
-    The single required field is ``type``; unknown fields are preserved as strings.
+    Unknown fields are preserved as strings. The ``type`` requirement is NOT applied
+    here — it belongs to concept files, not to the index entry point.
     """
     lines = path.read_text(encoding="utf-8").splitlines()
     if not lines or lines[0].strip() != "---":
@@ -64,30 +65,44 @@ def parse_concept_file(path: Path) -> ConceptFile:
         key, sep, value = line.partition(":")
         if sep:
             frontmatter[key.strip()] = _strip_matching_quotes(value.strip())
+    return frontmatter, "\n".join(lines[body_start:]).strip()
+
+
+def parse_concept_file(path: Path) -> ConceptFile:
+    """Parse frontmatter (leading ``---`` block, line-oriented ``key: value``) + body.
+
+    The single required field is ``type``; unknown fields are preserved as strings.
+    """
+    frontmatter, body = _parse_frontmatter_and_body(path)
     if "type" not in frontmatter:
         raise ValueError(f"{path.name}: frontmatter lacks the required field 'type'")
-    return ConceptFile(
-        path=path, frontmatter=frontmatter, body="\n".join(lines[body_start:]).strip()
-    )
+    return ConceptFile(path=path, frontmatter=frontmatter, body=body)
 
 
 def _parse_index_entry(index_path: Path) -> ConceptFile:
-    """Parse the bundle entry point, tolerating a frontmatter-less index (§3 Step 1).
+    """Parse the bundle entry point, where ``type`` is not required (§3 Step 1).
 
     method-spec §3 Step 1 renders the index as "the index body (the summary)" and
     treats every OTHER file as a "non-index concept file" (`## {type}: {title}`
-    section) — so the index is NOT a concept file, and it MAY omit frontmatter: a
-    generated index carrying only ``bundle_summary`` + cross-links (ingest spec §6,
-    frozen in the ingest golden) is valid. A frontmatter-ful index (the curated
-    convention, ``type: index``) still parses normally, its extra fields preserved.
-    When frontmatter is absent, the whole file is the body and ``type`` defaults to
-    ``index`` so downstream ``.type`` reads never raise.
+    section) — so the index is NOT a concept file, and the ``type`` requirement does
+    not reach it. It MAY omit frontmatter entirely (a generated index carrying only
+    ``bundle_summary`` + cross-links, ingest spec §6, frozen in the ingest golden),
+    and it MAY carry frontmatter that says nothing about ``type`` (an OKF v0.2
+    generated index stamps ``okf_version`` there). In both cases ``type`` defaults to
+    ``index`` so downstream ``.type`` reads never raise; other fields are preserved.
+    A curated index (``type: index``) is unaffected.
+
+    Keying this on the ABSENCE of a frontmatter block was the defect: a frontmatter-ful
+    index without ``type`` then raised out of ``parse_concept_file`` and killed the
+    whole navigation, not merely the index read.
     """
     text = index_path.read_text(encoding="utf-8")
     lines = text.splitlines()
-    if lines and lines[0].strip() == "---":
-        return parse_concept_file(index_path)
-    return ConceptFile(path=index_path, frontmatter={"type": _INDEX_TYPE}, body=text.strip())
+    if not lines or lines[0].strip() != "---":
+        return ConceptFile(path=index_path, frontmatter={"type": _INDEX_TYPE}, body=text.strip())
+    frontmatter, body = _parse_frontmatter_and_body(index_path)
+    frontmatter.setdefault("type", _INDEX_TYPE)
+    return ConceptFile(path=index_path, frontmatter=frontmatter, body=body)
 
 
 def navigate_bundle(bundle_dir: Path) -> list[ConceptFile]:
@@ -96,8 +111,8 @@ def navigate_bundle(bundle_dir: Path) -> list[ConceptFile]:
     Targets containing a path separator are out-of-bundle and skipped; resolution is
     boundary-checked against the bundle directory (fail-closed); broken links are
     skipped, never raised. Repeated links are de-duplicated. The index entry point
-    may omit frontmatter (``_parse_index_entry``); non-index concept files still
-    require ``type``.
+    does not require ``type`` — with or without a frontmatter block
+    (``_parse_index_entry``); non-index concept files still require it.
     """
     index_path = bundle_dir / _INDEX_FILENAME
     if not index_path.is_file():