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_test.ludic

@@ -46,6 +46,7 @@ function cmd_selfhost_test() -> int {
   sh_case("memsys", "65 66 0 99 12345 1")
   sh_case("math3", "45 90 180 -135 30 60 90")
   sh_case("transcend", "2718 1 148 1000 0 2303 1024 1414 3 250 0 1000 1016")
+  sh_case("vector", "3 4 5 4 6 2 2 11 2 6 999 999 999 1570 5 10")
   sh_case("textsplit", "1 1 1 3 1 1 1 1 1 1 1")
   sh_case("hash", "-2128831035 -468965076 114400290 114400290 0 -873187034 1095738169 0 1364076727 -2114883783 -845898438 -845898438 -78065325 -78057399 -3750763034362895579 -5808556873153909620 -4100651535478758590 -4100651535478758590 0 -5451962507482445012 7256831767414464289")
   sh_case("long", "1000000000000 1000000000001 1000000000005 3000000000000 1 1 1 -1000000000000 1000000 13")