Add type-blind note I/O and declared per-type record modules

[williamthorsen]

Add type-blind note I/O and declared per-type record modules

Problem

A single assertion-biased Frontmatter type forces both record shapes through assertion-shaped slots. The shared write path emits title/created/updated unconditionally; the shape of each record type is re-derived across the codebase.

Proposed solution

Introduce two layers, alongside the existing generic layer (nothing migrated yet):

• Type-blind note I/O: read(path) → { fields, body } and atomic write(path, fields, body) with correct YAML quoting/escaping, plus leaf validators isValidDate and asStringList.
• Declared per-type record modules KbAssertion and KbEvent, each owning parse/render/validate over the I/O layer. No shared base type; related only by the recordType discriminant value.

Part of the per-type record architecture epic (#848).

Acceptance criteria

Must have

• KbAssertion and KbEvent each parse, render, and validate their own field set.
• A KbEvent round-trips (parse → render → parse) carrying only its own fields — no title/created/updated injected.
• A KbAssertion round-trips unchanged.
• New behavior is covered by tests, including the event round-trip.