Capture the design decisions reached while grilling the core-parser spec:
- Parser is a total function; invalid frontmatter YAML surfaces through a
new Document AST `diagnostics` channel instead of throwing (ADR 0004).
- Parse all of OFM losslessly via `RawBlock`/`RawInline` fallbacks with
position-sliced verbatim source and a minimal plugin set (ADR 0005).
- Grammar-driven (not last-space) quantity/unit split, fixing the
mixed-number collision, with trimming and multi-word unit rules.
- Container blocks nest `Block[]` so annotations surface inside
blockquotes and list items; unified wikilink/transclusion shape.
Update SPEC.md and CONTEXT.md accordingly; no parser code yet.
