fix(ir): precise oversized-dim diagnostic on the alias path (0083)

The stateless alias-registration array-dim path collapsed foldDimU32's
distinct .too_large / .below_min outcomes into null, so an oversized type
alias (Big :: [5000000000]s64) emitted the FALSE 'an array dimension is not
a compile-time integer constant' message while the direct form correctly
reported 'array dimension 5000000000 does not fit in u32'.

Add program_index.reportDimError as the single source of dim-error wording
(the stateful path now emits through it too) and type_bridge.foldArrayDim to
surface the DimU32 reason at the alias-registration site. An oversized/negative
alias dim now routes to reportDimError for the same precise message as the
direct form; a genuinely non-const alias dim keeps the alias-specific message.

Regression: examples/1131-diagnostics-array-dim-oversized-u32-alias.sx

examples/1131-diagnostics-array-dim-oversized-u32-alias.sx

@@ -0,0 +1,24 @@
+// An array dimension that folds to a valid compile-time integer but exceeds a
+// `u32` is a hard error — and it must report the SAME precise diagnostic whether
+// the array is written directly (`a : [5_000_000_000]s64`, see example 1130) or
+// behind a type ALIAS (`Big :: [5_000_000_000]s64`, here). Both forms now route
+// the dimension through one shared folder + one shared message map, so they
+// cannot diverge.
+//
+// Regression (issue 0083 / F0.4 attempt 7): the stateless alias-registration
+// path collapsed `foldDimU32`'s distinct `.too_large` outcome into `null` and
+// emitted ONE generic "an array dimension is not a compile-time integer
+// constant" message — FALSE, since 5_000_000_000 IS a compile-time integer
+// constant; it merely doesn't fit a `u32`. The alias path now consults the
+// shared fold and emits the precise "does not fit in u32" message, matching the
+// direct form. (A genuinely non-const alias dim still gets the generic message —
+// see example 1129.)
+#import "modules/std.sx";
+
+Big :: [5000000000]s64;
+
+main :: () {
+    a : Big = ---;
+    a[0] = 7;
+    print("unreachable: {}\n", a[0]);
+}

examples/expected/1131-diagnostics-array-dim-oversized-u32-alias.exit

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

examples/expected/1131-diagnostics-array-dim-oversized-u32-alias.stderr

@@ -0,0 +1,5 @@
+error: array dimension 5000000000 does not fit in u32
+  --> examples/1131-diagnostics-array-dim-oversized-u32-alias.sx:18:9
+   |
+18 | Big :: [5000000000]s64;
+   |         ^^^^^^^^^^

examples/expected/1131-diagnostics-array-dim-oversized-u32-alias.stdout

@@ -0,0 +1 @@
+