fix(ir): unify named-const array-dim resolution + kill length-0 fabrication (0083)

A type alias whose dimension is a named const (`Arr :: [N]T`) resolves its
dimension eagerly during scanDecls pass 1, on the stateless registration path,
which can only read `module_const_map`. Typed consts (`N : s64 : 16`) register
only in pass 2 and a forward-declared untyped const had not registered yet, so
the stateless resolver saw an empty table, printed a non-fatal warning,
fabricated length 0, and continued — yielding a 0-byte alloca, garbage reads,
and a segfault for slice/struct elements.

- scanDecls pass 0 pre-registers every integer-valued module const before any
  type alias resolves, so typed, untyped, and forward-referenced consts all
  resolve identically.
- Both dim resolvers now share `program_index.moduleConstInt`, so the stateful
  body-lowering path and the stateless registration path cannot diverge.
- `resolveArrayLen` returns `?u32`; `resolveCompound` yields `.unresolved` on
  null instead of a 0-length array. The stateful path emits a diagnostic; the
  alias-registration path surfaces an unresolved alias as a clean compile error
  that aborts the build. The Vector lane-count `else => 0` is fixed the same way.

Regressions: examples/0143 (typed-const dim direct + via alias for s64/string/
struct, forward-ref alias, nested) and examples/1129 (an unresolvable computed
dim halts with a clean diagnostic + non-zero exit). Both fail on the pre-fix
compiler (garbage/segfault; warning+exit0) and pass after.

examples/expected/0143-types-typed-const-array-dim.exit

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

examples/expected/0143-types-typed-const-array-dim.stderr

@@ -0,0 +1 @@
+

examples/expected/0143-types-typed-const-array-dim.stdout

@@ -0,0 +1,6 @@
+direct d0=3 d7=21 len=8
+alias a0=7 a7=99 len=8
+alias s0=hi s7=yo
+alias p0x=1 p0y=2 p7x=5
+nested g00=1 g77=10
+fwd f0=4 f4=40 len=5

examples/expected/1129-diagnostics-array-dim-not-const.exit

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

examples/expected/1129-diagnostics-array-dim-not-const.stderr

@@ -0,0 +1,5 @@
+error: type alias 'BadArr' could not be resolved: an array dimension is not a compile-time integer constant
+  --> examples/1129-diagnostics-array-dim-not-const.sx:14:11
+   |
+14 | BadArr :: [M + 1]s64;
+   |           ^^^^^^^^^^

examples/expected/1129-diagnostics-array-dim-not-const.stdout

@@ -0,0 +1 @@
+