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

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