fix(ir): evaluate constant-expression array dimensions (0083)

A constant-FOLDABLE expression array dimension (`[M + 1]`, `[M * N]`,
`[N - M]`, nested `[M + N - 1]`, parenthesised `[(M + 1) * 2]`, mixing
untyped and typed module consts) was wrongly rejected as "not a
compile-time integer constant" even though every operand is
compile-time-known. Attempts 1-3 resolved only a bare named-const dim or
a literal; an expression dim must be EVALUATED, not rejected.

Fix: the shared dim resolver now routes the dimension through a single
constant integer-expression evaluator (`program_index.evalConstIntExpr`)
that folds integer `+ - * / %` and unary negate over literals and
named/typed module consts, recursively (parentheses carry no AST node).
The leaf-name lookup is delegated via `ctx.lookupDimName`, so the
stateful body-lowering path (`Lowering`, which also sees comptime
constants and generic `$N` values) and the stateless registration path
(`type_bridge.StatelessInner`, module consts only) share the EXACT SAME
folding logic and cannot diverge — an expression dim via a type alias
resolves identically to the direct form.

No-fabrication discipline unchanged: a genuinely non-comptime dimension
(runtime local, non-comptime call, unbound name) or arithmetic that
overflows / divides by zero still yields null -> `.unresolved` -> the
same clean compile-halting diagnostic, never a fabricated length.

- examples/0144-types-const-expr-array-dim.sx: every expression form,
  direct vs alias, scalar / string / struct element types (fails on the
  pre-fix compiler, passes after).
- examples/1129 re-pointed at a genuinely non-const dimension
  (`[get()]s64`, a runtime call) so it still proves the stateless
  clean-halt (a foldable expression is no longer an error).
- program_index.test.zig: unit test for evalConstIntExpr folding and
  clean-halt-on-non-const.

examples/expected/0144-types-const-expr-array-dim.exit

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

examples/expected/0144-types-const-expr-array-dim.stderr

@@ -0,0 +1 @@
+

examples/expected/0144-types-const-expr-array-dim.stdout

@@ -0,0 +1,7 @@
+add direct.len=5 alias.len=5 d4=7 a4=7
+mul direct.len=24 alias.len=24 d23=230 a23=230
+sub direct.len=2 alias.len=2 d1=9 a1=9
+nest direct.len=9 alias.len=9 paren direct.len=10 alias.len=10
+typed direct.len=6 alias.len=6
+str alias.len=5 s0=hi s4=yo
+struct alias.len=5 p0x=1 p4y=6

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

@@ -1,5 +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
+  --> examples/1129-diagnostics-array-dim-not-const.sx:18:11
    |
-14 | BadArr :: [M + 1]s64;
+18 | BadArr :: [get()]s64;
    |           ^^^^^^^^^^