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.

SPEC.md

@@ -56,12 +56,14 @@ Multi-word names work naturally: `@unsalted butter{30 g}`.
 - Simple fraction: `1/2`
 - Mixed number: `1 1/2`
 
-**Unit**: optional, free-form string.
+**Unit**: optional.
 The quantity/unit split is **grammar-driven**: the parser matches the quantity greedily against the quantity grammar (mixed number → fraction → decimal → integer) anchored at the start of the braces, and any non-empty remainder after the delimiting space is the unit.
 `@butter{1 1/2 tbsp}` → quantity `1 1/2`, unit `tbsp`.
 `@butter{1 1/2}` → quantity `1 1/2`, no unit.
 A plain "last space" rule cannot express this, because a mixed-number quantity itself contains a space; the grammar is authoritative.
 If the brace content does not begin with a grammar-valid quantity (e.g. `@stock{a splash}`), the entire content is preserved as the quantity string and the unit is left empty, so no annotation is dropped.
+The unit is **normalised**: a unit matching a known alias is rewritten to its canonical abbreviation (case-insensitive), and any unit not in the table is passed through verbatim.
+See the [Units](#units) section for the alias table.
 
 **Examples**:
 ```
@@ -90,7 +92,7 @@ $name{quantity unit}
 
 **Quantity**: optional. Follows the same format rules as Ingredient quantity (integer, decimal, fraction, mixed number).
 
-**Unit**: optional, free-form. Same grammar-driven quantity/unit split as Ingredient.
+**Unit**: optional. Same grammar-driven quantity/unit split and canonical [unit normalisation](#units) as Ingredient.
 
 **Examples**:
 ```
@@ -116,7 +118,7 @@ $cupcake liner{12 large}
 **Range**: two quantities separated by `-`. Since negative quantities do not exist, the `-` is unambiguous.
 `~1/2-1 hr` → range of 1/2 hr to 1 hr.
 
-**Unit**: one word from the known set, normalised to its canonical abbreviation:
+**Unit**: one word from the known set, matched **case-insensitively** and normalised to its canonical abbreviation:
 
 | Written | Canonical |
 |---------|-----------|
@@ -139,6 +141,36 @@ A `~` followed by a number but not a known unit word is not a valid annotation a
 
 ---
 
+## Units
+
+Ingredient and Cookware units are normalised: a unit matching a known alias is rewritten to its canonical abbreviation, and any unit not in the table is passed through verbatim (its original casing and spacing preserved).
+Lookup is **case-insensitive**, and multi-word aliases (`fluid ounce`) match on the whole unit string.
+Only the canonical unit is retained; the author's original spelling is not kept.
+
+| Canonical | Aliases |
+|-----------|---------|
+| `g`     | g, gram, grams |
+| `kg`    | kg, kilogram, kilograms, kilo, kilos |
+| `mg`    | mg, milligram, milligrams |
+| `oz`    | oz, ounce, ounces |
+| `lb`    | lb, lbs, pound, pounds |
+| `ml`    | ml, milliliter, millilitre, milliliters, millilitres |
+| `l`     | l, liter, litre, liters, litres |
+| `tsp`   | tsp, teaspoon, teaspoons |
+| `tbsp`  | tbsp, tablespoon, tablespoons |
+| `cup`   | cup, cups |
+| `fl oz` | fl oz, fluid ounce, fluid ounces |
+| `pt`    | pt, pint, pints |
+| `qt`    | qt, quart, quarts |
+| `gal`   | gal, gallon, gallons |
+
+Single-letter cooking abbreviations (`t`, `T`, `c`) are intentionally excluded as ambiguous.
+Count and descriptive units (`clove`, `pinch`, `dash`, `can`, `large`, `to taste`) have no canonical form and pass through unchanged.
+
+Timer units are normalised from their own known set (see [Timer](#timer)), also case-insensitively.
+
+---
+
 ## Recipe structure
 
 A KitchenMD file is prose instructions only.