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

src/ir/program_index.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const ast = @import("../ast.zig");
 const types = @import("types.zig");
 const inst = @import("inst.zig");
+const errors = @import("../errors.zig");
 
 const Node = ast.Node;
 const TypeId = types.TypeId;
@@ -147,6 +148,24 @@ pub fn foldDimU32(node: *const Node, ctx: anytype, min: u32) DimU32 {
     return .{ .ok = @intCast(v) };
 }
 
+/// THE single source of array-dimension diagnostic wording. Both array-dim
+/// resolvers — the stateful body-lowering path (`Lowering.resolveArrayLen`) and
+/// the stateless registration-time path (the alias-registration site, via
+/// `type_bridge.foldArrayDim`) — emit through here, so an oversized / negative /
+/// non-const dimension reports the SAME message regardless of whether it was
+/// written directly (`a : [N]T`) or via a type alias (`Arr :: [N]T`). Folding
+/// the wording into one place is the diagnostic-accuracy half of the issue-0083
+/// unify-or-diverge story: `foldDimU32` is the single fold, this is the single
+/// message map. Only call with a non-`.ok` result (the `.ok` arm is a no-op).
+pub fn reportDimError(diag: *errors.DiagnosticList, span: ?ast.Span, result: DimU32) void {
+    switch (result) {
+        .ok => {},
+        .below_min => |v| diag.addFmt(.err, span, "array dimension must be non-negative, got {}", .{v}),
+        .too_large => |v| diag.addFmt(.err, span, "array dimension {} does not fit in u32", .{v}),
+        .not_const => diag.addFmt(.err, span, "array dimension must be a compile-time integer constant", .{}),
+    }
+}
+
 pub const GlobalInfo = struct { id: inst.GlobalId, ty: TypeId };
 
 /// Single lowering access point for declaration-name / import / visibility