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>

tools/x/selfhost.ludic

@@ -25,6 +25,7 @@ function selfhost_frags() -> []pointer {
   push(f, "selfhost/emit_intrin.ludic")
   push(f, "selfhost/emit_intrin2.ludic")
   push(f, "selfhost/emit_math.ludic")
+  push(f, "selfhost/emit_vector.ludic")
   push(f, "selfhost/emit_text.ludic")
   push(f, "selfhost/emit_hash.ludic")
   push(f, "selfhost/emit_list.ludic")