docs: record the measured generator coverage of the new kind_line rule

context/code/formula-and-io.md already says a new grammar production needs
generator coverage or the proptests quietly stop describing the format.
`kind_line` is an alternative of `category_entry`, so the grammar walk
reaches it with no generator change — but that is precisely the claim the
section exists to say must be checked rather than assumed.

Probed over 500 files from generator::improv_file(): 14 carried a
`kind: label` line and all 14 parsed to a CategoryKind::Label, so each
256-case proptest run exercises the production about seven times.

The probe is not kept. Its rate depends on proptest's internal RNG, so as
a standing assertion it would fail on a dependency bump for a reason that
has nothing to do with the format — the measurement belongs in the
argument, where it can be re-run and corrected.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

context/code/formula-and-io.md

@@ -207,6 +207,17 @@ One case the format already allowed and this had to answer: two
 merged in pass 1. Absence is not a claim, so the merge keeps a stated kind
 rather than letting a later kindless section erase it.
 
+**The generator coverage this section demands was measured, not assumed.**
+`kind_line` is an alternative of `category_entry`, which the grammar walk
+reaches generically, so nothing in the generator needed changing — but that is
+the kind of claim this section exists to say must be checked. Probed with a
+throwaway test over 500 files from `generator::improv_file()`: **14 carried a
+`kind: label` line and all 14 parsed to a `CategoryKind::Label`**, so at the
+default 256 cases each proptest run exercises the new production about seven
+times. The probe was removed rather than kept: its rate depends on proptest's
+internal RNG, so as a permanent assertion it would fail on a dependency bump
+for a reason unrelated to the format.
+
 ### The tree walkers are infallible by construction
 
 Every `build_*` in `parser.rs` operates on a `Pair` pest has already