feat(stdlib): 2D Vector type + Vector.* namespace (issue #25)

Implement the Vec.* half of #25 under the proper (de-abbreviated) name
Vector, unblocking it with a self-contained value type instead of waiting
on the full #1 type system.

A Vector is two Q16.16 fixed components (x, y) packed into one i64 — a true
by-value type that lives in a register and never allocates (reuses the new
`long`/i64 support; llty maps `Vector` to i64). Fifteen operations, all
deterministic fixed-point reusing fx_mul/fx_div/fx_lerp and the @fn_fx_*
prelude: make/zero/x/y, add/sub/scale/dot, length/distance/normalize/lerp,
rotate/angle/from_angle.

New selfhost/emit_vector.ludic (wired into emit_ns_call + the frag list),
the `Vector` primitive type in llty and the grammars/LSP/JetBrains tokens,
docs (type-vector + 15 Vector.* pages + section), and a registered test.
Reseeded; C-free fixpoint holds; all suites green (45/25/29); site + check.py OK.

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

docs/language/vector/vector-angle.md

@@ -0,0 +1,22 @@
+---
+id: vector-angle
+name: Vector.angle
+category: vector
+kind: namespace-method
+tokens: Vector.angle
+sig: Vector.angle(v) -> fixed
+tip: The angle of a vector in radians.
+order: 13
+ns: Vector
+member: angle
+---
+
+Returns the angle of <code>v</code> in radians, measured from +x, in the range -pi..pi (via <code>atan2</code>). Feed it back into <code>Vector.from_angle</code> or <code>Vector.rotate</code>.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let heading = Vector.angle(velocity)
+  }
+}
+```