fix(ir): route every comptime-int through the shared evaluator (0083)

Attempts 1–4 fixed the array-dimension paths but the same length-0
fabrication class survived on every other site that resolves a
compile-time integer. Unify them all on the single shared
`program_index.evalConstIntExpr` so they cannot diverge:

- All three Vector lane resolvers (resolveTypeCallWithBindings,
  resolveParameterizedWithBindings, resolveArrayLiteralType) and both
  generic value-param binders (instantiateGenericStruct,
  instantiateTypeFunction) hand-rolled an `else => 0` switch. A
  module-const lane `Vector(N, f32)` fabricated a 0-lane `<0 x float>`
  (LLVM "huge alignment" abort); a value-param `Vec(N, f32)` fabricated
  a 0 binding / wrong mangled name. They now fold through the shared
  evaluator and emit a clean diagnostic + `.unresolved` on a non-const
  operand (resolveVectorLane / resolveValueParamArg) — never 0.
- evalComptimeInt (inline-for bounds) delegated to the shared evaluator,
  so `inline for 0..M` / `0..(M+1)` fold like array dims. The `<pack>.len`
  leaf moved into the shared folder via a new `ctx.lookupPackLen`.
- The unknown-type semantic checker no longer walks a value-param
  position (`Vector(N, …)` / `Vec(N, …)`) as a type name (was reporting
  "unknown type 'N'").
- The parameterized-type-arg parser and the function-body lookahead
  (hasFnBodyAfterArrow) accept a const-EXPRESSION in a value position, so
  `Vector(M + 1, f32)` and `[M + 1]T` parse as a return type too (the
  latter a pre-existing array-dim sibling that the same heuristic broke).

Regressions: examples/1501 (named-const + const-expr lane, direct +
alias, 3/4-lane reads), 1502 (runtime lane clean-halts, exit 1, no LLVM
crash), 0207 (Vec(N)/Vec(M+1) == Vec(3) instantiation), 0610 (inline-for
const bounds). Shared-evaluator unit test extended with the pack-len arm.

zig build && zig build test && bash tests/run_examples.sh: 395 passed,
0 failed.

examples/expected/0207-generics-value-param-const.exit

@@ -0,0 +1 @@
+0

examples/expected/0207-generics-value-param-const.stderr

@@ -0,0 +1 @@
+

examples/expected/0207-generics-value-param-const.stdout

@@ -0,0 +1,3 @@
+named: len=3 a0=10.000000 a2=30.000000
+expr: len=3 e2=9.000000
+copy: len=3 b2=30.000000

examples/expected/0610-comptime-inline-for-const-bound.exit

@@ -0,0 +1 @@
+0

examples/expected/0610-comptime-inline-for-const-bound.stderr

@@ -0,0 +1 @@
+

examples/expected/0610-comptime-inline-for-const-bound.stdout

@@ -0,0 +1,2 @@
+sum 0..M = 3
+sum 0..(M+1) = 6

examples/expected/1501-vectors-const-lane.exit

@@ -0,0 +1 @@
+0

examples/expected/1501-vectors-const-lane.stderr

@@ -0,0 +1 @@
+

examples/expected/1501-vectors-const-lane.stdout

@@ -0,0 +1,4 @@
+direct3: 1.000000 2.000000 3.000000
+direct4: 1.000000 2.000000 3.000000 4.000000
+alias3: 7.000000
+alias4: 8.000000

examples/expected/1502-vectors-runtime-lane-not-const.exit

@@ -0,0 +1 @@
+1

examples/expected/1502-vectors-runtime-lane-not-const.stderr

@@ -0,0 +1,11 @@
+error: Vector lane count must be a positive compile-time integer constant
+  --> examples/1502-vectors-runtime-lane-not-const.sx:14:16
+   |
+14 |     v : Vector(lanes(), f32) = ---;
+   |                ^^^^^^^
+
+error: field 'x' not found on type 'unresolved'
+  --> examples/1502-vectors-runtime-lane-not-const.sx:15:32
+   |
+15 |     print("unreachable: {}\n", v.x);
+   |                                ^^^

examples/expected/1502-vectors-runtime-lane-not-const.stdout

@@ -0,0 +1 @@
+