docs(k3): round 22, frontmatter a YAML reader reads back the same

K3-22's report, CHANGELOG under [Unreleased], and two corrections of a
sentence measured false.

README and CLAUDE.md both said of the flow form "both are valid YAML, and a
real YAML consumer recovers the same structure from either". An unquoted URL
with a query string inside a flow mapping is not valid YAML for PyYAML, and
the quoted form is refused by the pinned guard; the paragraph now states the
limit. CLAUDE.md also records the K3-22 invariant and the new refusal of a
`--frontmatter` flow value with such a leaf.

Measured, fix `ed0418f` against base `0308169`, each tree built twice from
frozen exports: five-document project 0 files moved; R761 XML and HTML 1 line
each; K2 default 42 `title` lines, after which 454 of 454 frontmatters parse
and read back the same (base 413 parse, 412 read back). hit@k on R761 base =
fix, S1-S6 6/6 at hit@1/8/50 at both k, KP rank 1; okf check 32 of 32 payloads
0 findings over 16 rules; K2 pin 7 passed in the export.

Found outside the order: the pinned guard refuses nearly every segmented
concept okf writes, on its scalar flow sequences (`source_offset`,
`references`, `derived`) -- identical before and after, so older than K3-22.

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

CHANGELOG.md

@@ -7,6 +7,38 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- **Frontmatter this library writes is YAML a YAML reader reads back the same
+  (K3-22).** SPEC § 11 point 1 requires "a parseable YAML frontmatter block" in
+  every file. Measured with PyYAML 6.0.3 before the change, the pinned K2
+  default bundle failed `safe_load` on 41 of 455 blocks (and a 42nd parsed to
+  a truncated title), and each R761 build on 1 -- every one a block scalar
+  written verbatim: a title with `": "` or `" #"`, a leading `- `, `*` or `**`,
+  a trailing `:`.
+  - **Block scalars:** a value K3-19's plain-scalar rule refuses is written
+    double-quoted, `\` and `"` escaped; every other value keeps its bytes.
+    Rebuilt, the five-document project moves 0 files, each R761 build 1 line
+    and the K2 default bundle 42 `title` lines, after which all 454 of its
+    frontmatters parse and read back the same.
+  - **Flow leaves (`sources`, Door A's list, a run-stated flow value):** the
+    pinned guard refuses any quote inside a flow mapping, so a leaf PyYAML
+    would refuse (`?`, `,[]{}`, `": "`, `" #"`, a quote, a leading indicator)
+    has no form both read and is refused with the door's existing code
+    (`inbox_source_file_unaddressable`, `inbox_source_title_unaddressable`,
+    `source_reference_unquotable`, `run_frontmatter_invalid`).
+  - **Behaviour change:** `okf build --frontmatter 'sources=[{ resource: <URL
+    with a query string>, … }]'` now exits 2 and writes nothing. K3-19's own
+    flagged R761 build used such a URL and wrote 2 761 of 2 761 frontmatters
+    PyYAML refuses.
+  - **Readers** (`parse_frontmatter`, the index and structure readers, both
+    `read_sources` branches) unquote a `"`-wrapped value; `'`-wrapped values
+    are untouched. On 25 273 files of existing bundles the readers return
+    exactly what they returned before.
+  - The generated `SKILL.md` header goes through the same block rule.
+  - PyYAML joins the `dev` dependency group only; `src/` imports no yaml.
+  - Report: `docs/2026-09-11-k3-runde22-yaml-lesbar-frontmatter.md`.
+
 ## [0.8.3] — 2026-09-11
 
 ### Added