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>

CLAUDE.md

@@ -301,14 +301,30 @@ specifies it. Because "always latest" decays silently, the release checklist
 carries an upstream-version re-check.
 
 **Structured frontmatter values are emitted in YAML *flow* form, never block.**
-Both are valid YAML and an upstream reader recovers the same structure from
-either, but this library's parser is line-oriented: it round-trips a flow
+Both can be valid YAML -- within the flow-scalar limit below -- and an upstream
+reader then recovers the same structure from either, but this library's parser
+is line-oriented: it round-trips a flow
 mapping as an opaque value and cannot read the block form at all — two block
 mappings sharing an inner key (§10.2's `executor` and `attester`, both carrying
 `resource`) collapse into one namespace and the first is lost silently.
 Emitting block would produce bundles we cannot read back. Reading it needs the
 structured reader (D1b); until then the constraint binds what we write.
 
+**Every value is written so a YAML reader reads it back the same (K3-22).**
+SPEC § 11 point 1 requires "a parseable YAML frontmatter block", and before
+K3-22 the pinned K2 default bundle failed PyYAML on 41 of 455 blocks and each
+R761 build on 1 -- block scalars written verbatim. A block scalar that is not
+plain-safe (`profiles.yaml_block_plain`, K3-19's rule) is now written
+double-quoted with `\` and `"` escaped; every other value keeps its bytes. A
+FLOW leaf has no quoted form -- the pinned guard refuses any quote inside a
+flow mapping -- so a leaf PyYAML would refuse or misread (`?`, `": "`, `" #"`,
+a quote, a leading indicator) is refused by `profiles.yaml_flow_plain` with the
+door's existing code, never written. Readers unquote a `"`-wrapped value only:
+0 such values existed in any measured bundle, while 11 193 `'`-wrapped ones do
+and stay untouched. PyYAML is a dev dependency that validates the rules in
+`tests/test_yaml_frontmatter.py`; `src/` imports no yaml. Report:
+`docs/2026-09-11-k3-runde22-yaml-lesbar-frontmatter.md`.
+
 **Every upstream release runs `docs/upstream-okf-upgrade-runbook.md`.** Pin the
 commit, enumerate the whole `okf/` tree, **read the shipped example bundles and not
 only `SPEC.md`**, classify the diff, measure our exposure and each consumer's, plan
@@ -587,9 +603,13 @@ and fixtures, never code.
   `docs/2026-09-08-k3-runde3-per-filtype.md`.
 - **`--frontmatter KEY=VALUE` (K3-19, repeatable) is not a segmentation flag**
   and moves no byte unless given: it stamps a key on every concept of the run,
-  split on the FIRST `=` and written verbatim on ONE line -- a block-form
+  split on the FIRST `=` and written on ONE line -- a block-form
   `sources` is invisible to `parse_frontmatter`, so the flow form is the only
-  one that survives our own readers. It adds any key and REPLACES only
+  one that survives our own readers. Since K3-22 a scalar goes out
+  double-quoted where a YAML reader would not read it plain, and a flow value
+  goes out as given but is REFUSED (exit 2) when a leaf has no flow form both
+  PyYAML and the guard read -- so a `sources` URL with a query string, the form
+  K3-19's own flagged build wrote on 2 761 of 2 761 concepts, fails the build. It adds any key and REPLACES only
   `sources` and `description`, the two with a derived layer below them:
   precedence flag > what the document declares > file name. Every other key
   the door writes (`inbox._door_keys`, including Door A's `ingest_manifest`,