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>

selfhost/emit_core.ludic

@@ -73,6 +73,7 @@ function lbl(pfx: pointer) -> pointer { let r = (pfx + itoa(ll_lbl));  ll_lbl =
 function llty(t: pointer) -> pointer {
   if (t == "int") or (t == "bool") or (t == "fixed") or (t == "entity") { return "i32" }   # entity = an i32 handle (self())
   if (t == "long") { return "i64" }               # a 64-bit signed integer
+  if (t == "Vector") { return "i64" }             # a 2D vector: (x, y) fixeds packed into one i64
   if (t == "byte") { return "i8" }                # a single byte (p[i] on a raw ptr)
   if (t == "words") or (t == "fixeds") or (t == "pointers") { return "ptr" }   # typed buffers
   if (t == "void") { return "void" }