test(frontmatter): what okf writes must be YAML a YAML reader reads back the same

K3-22, red first. SPEC SS 11 point 1 requires "a parseable YAML frontmatter
block" in every file. Measured with PyYAML 6.0.3 before any code moved: the
K2 default bundle this repository pins fails safe_load on 41 of 455
frontmatter blocks, all on `title` (a leading `- `, `**` or `*`, or ": "),
and the R761 build on 1 of 2 763 (a title ending in `:`). No `sources` value
okf itself wrote failed; the 4 605 consumer failures come from that
consumer's own writer.

Each case goes through a public path (render_inbox_concept, the profile
emitter, Door A's sources renderer, skill.render) and is held to three
readers: safe_load must not raise, BaseLoader must return the same strings
as parse_frontmatter / read_sources, and the pinned guard must admit it. The
guard is why quoting inside a flow mapping is not the fix: 1.3.0 refuses any
quote in a flow mapping (measured), so a `sources` leaf PyYAML needs quoted
has no form both readers accept, and is refused instead.

57 of 85 red on 0308169; the 28 green are the known-negatives and controls.
The rest of the suite is unchanged: 1695 passed (1667 + 28), 1 skipped.

pyyaml joins [dependency-groups] dev and nothing else; uv.lock moves by
exactly the two lines that dev dependency adds.

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

uv.lock

@@ -569,6 +569,7 @@ ocr = [
 dev = [
     { name = "mypy" },
     { name = "pytest" },
+    { name = "pyyaml" },
     { name = "ruff" },
 ]
 
@@ -587,6 +588,7 @@ provides-extras = ["extract", "ocr"]
 dev = [
     { name = "mypy", specifier = ">=1.14" },
     { name = "pytest", specifier = ">=8" },
+    { name = "pyyaml", specifier = ">=6.0.3,<7" },
     { name = "ruff", specifier = ">=0.16.6,<0.17" },
 ]