hyperguild#138: internal/mcp/tools_notes.go's frontmatterBlock, internal/api/frontmatter.go's parseFrontmatter, and internal/frontmatter/frontmatter.go's split each independently detected a note's --- fence, converging on the same shape through parallel bug-fixing across today's #123/#126/#128 sagas. Each carried a subtly different gap: internal/frontmatter's closing-fence match had no trailing-newline requirement (a document ending "\n---extra" was treated as validly closed); internal/mcp needed CRLF/BOM guards bolted on across two separate review rounds after both silently defeated its own HasPrefix check; internal/api never gained either fix, so the same encoding-dependent bypass was live there too, just never triggered by a test. internal/frontmatter is the leaf package (imports nothing internal), so it becomes the canonical fence detector: exported Split(doc []byte) (block, rest []byte, ok bool), BOM-stripped and CRLF-normalized, with the stricter trailing-newline-required close. internal/api and internal/mcp now call it instead of re-detecting the fence themselves, keeping only what's genuinely package-specific (api's ordered fmLine round-trip shape, mcp's map + block- scalar-fold logic). Found two real regressions while wiring the callers, caught by each package's own pre-existing byte-exactness tests going red immediately: Split's block/rest boundary sits one byte differently than either caller's prior contract (it keeps the fence-adjacent newline on each side, by design for Fix's own reconstruction) — both callers now TrimSuffix/TrimPrefix exactly that one byte to restore their exact prior semantics. Neither regression would have been visible without those tests already existing. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>