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/docgen/inventory.json

@@ -329,7 +329,8 @@
     "type-slices",
     "type-string",
     "type-void",
-    "type-words"
+    "type-words",
+    "type-vector"
   ],
   "world": [
     "world-get",
@@ -348,5 +349,22 @@
     "world-attach",
     "world-detach",
     "world-query_next"
+  ],
+  "vector": [
+    "vector-make",
+    "vector-zero",
+    "vector-x",
+    "vector-y",
+    "vector-add",
+    "vector-sub",
+    "vector-scale",
+    "vector-dot",
+    "vector-length",
+    "vector-distance",
+    "vector-normalize",
+    "vector-lerp",
+    "vector-rotate",
+    "vector-angle",
+    "vector-from_angle"
   ]
-}
+}