feat(types): IVec2 + Rect 2D value types (#1)

Phase 1 of the fuller type-system proposal: two by-value spatial types
that lower to packed integers (no heap, copy like scalars).

- IVec2 — integer 2D vector, a pair of int packed into one i64, for tile
  and grid coordinates: make/zero/x/y/add/sub/scale/dot, the grid distance
  manhattan, equal, and to_vector (widen into the fixed-point Vector).
- Rect — axis-aligned rectangle, four Q16.16 fixed components packed into
  one i128, for HUD boxes and hitboxes: make/x/y/w/h, the derived
  right/bottom/center, and the contains (point) / intersects (overlap) tests.

Both are exact and deterministic, bit-identical on every platform. Vector
and Color already cover phase 1's other 2D primitives.

Wired end to end: emit_core llty (IVec2->i64, Rect->i128), emit_call
dispatch, the FRAGS list + reseeded seed, a selfhost test (types2d),
per-symbol docs + type pages + inventory, and the vocabulary/editor sync
(header, JetBrains, TextMate, LSP).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/ivec2/_section.md

@@ -0,0 +1,7 @@
+---
+id: ivec2
+title: IVec2
+order: 8
+---
+
+Integer 2D vector math for tile and grid coordinates, cell offsets, and integer sizes. An <code>IVec2</code> is a pair of whole-number <code>int</code> components (x, y) packed into one value, so it is copied by value and never allocates. Every operation is exact integer arithmetic — no rounding, and bit-identical on every platform. Use it wherever a fractional part would be meaningless; reach for <code>Vector</code> when you need sub-pixel precision. Arguments are positional.