test(okf): pin the two boundary classes the new carriers introduced

Probed after 1.3.0 landed, on advisor challenge. Both came back clean --
these pin them so they cannot regress silently.

1. A `- ` item whose text begins with a YAML indicator AND carries a `": "`
   could in principle route into the block-mapping path with a
   half-validated key instead of into `_reject_dangerous_value`. It does
   not: `&anchor id` fails `_KEY_RE` on the key side, so the item falls
   through intact and raises on the indicator. One row each for & * ! %
   backtick, plus the merge key inside an entry.

2. `_consume_block_mapping` ends at a blank line and at column zero, and
   hands an index back to `_consume_block_list`, which hands one back to
   `_parse_flat`. A line orphaned by that boundary -- a `resource:` left
   over from a mapping that closed early -- must RAISE, not be dropped. A
   pointer that vanishes rather than failing is this repo's failure class,
   so it gets a test rather than a probe. Plus the return-index contract:
   a top-level key after a multi-entry list is neither swallowed nor
   re-read as an item.

868 green (was 859), 130/130 classes. Test-only; no source change.

docs/ADOPTION-BRIEF.md

@@ -144,7 +144,7 @@ python -m llm_ingestion_guard.coverage    # exit 0 = all as documented
 
 As of `v1.3.0`: **130 / 130 defended classes demonstrated (recall 100%)** and **6 /
 6 documented gaps still hold** (a *closed* gap fails the test, forcing a doc
-update). The matrix is the single source of truth for the test suite (**859
+update). The matrix is the single source of truth for the test suite (**868
 passing**), which also asserts total recall, that every lexicon pattern has a
 case (so the matrix cannot fall behind the lexicon), the full LLM02 secret-egress
 set, and the container-layer front-end (CSV formula-injection, zip-slip/bomb,