docs: sync core-parser spec and add unit normalisation

Bring core-parser.md in line with the resolved design in CONTEXT.md and
ADRs 0004/0005, and promote a new decision to normalise ingredient/cookware
units.

- Full sync of the core-parser spec: diagnostics/total-function channel,
  container blocks (ListItemBlock, BlockquoteBlock), RawInline alongside
  RawBlock via position-slicing, the unified WikilinkNode/TransclusionNode
  shape as two typed nodes, and the minimal remark plugin set.
- New ADR 0006: normalise ingredient/cookware units via a known-alias table
  with passthrough, case-insensitive, canonical-only; align Timer matching
  to case-insensitive too.
- SPEC.md: units are normalised (new Units section, alias table); Timer table
  marked case-insensitive.
- CONTEXT.md: add Unit Normalisation entry, resolve the open item.
- fixtures/basic.md: add a blockquote (callout) carrying an annotation and a
  non-canonical unit so the integration test exercises containers and
  normalisation.
- Expand the core test.todo checklists to cover the new behaviour (containers,
  raw fallbacks, diagnostics, unit normalisation, wikilink/transclusion
  variants); still pending, suite stays green.

fixtures/basic.md

@@ -24,6 +24,9 @@ Gradually whisk in @milk{300 ml} until you have a smooth, lump-free batter.
 Melt @unsalted butter{30 g} in a $non-stick frying pan{}, then stir most of it into the batter, reserving a little for the pan.
 Season with a pinch of @black pepper{} if you like a savoury edge.
 
+> [!tip] Fluffier pancakes
+> For a lighter texture, whisk in @caster sugar{2 tablespoons} and let the batter rest for ~5 mins before cooking.
+
 ### Cooking
 
 1. Set the $non-stick frying pan{} over a medium-high flame and let it heat for ~1 min.