fix(R1): own const_decl pending alias wins over flat imports (issue 0107)

When a module declares `A :: B; B :: u64;` and both a flat import and a
namespaced import export `B :: u8`, the flat import's B was discovered by
flatTypeAuthorCount before the own B :: u64 was processed — binding A to
u8 and silently truncating values.

Fix: ownConstDeclIsPendingAlias guard added to selectNominalLeaf between
the own-alias check and the flat-import walk. If the querying module has
an own const_decl for the name that is not yet in type_aliases_by_source,
return .pending so the forward-alias fixpoint resolves it correctly.

Regression: examples/0830-modules-flat-ns-same-name-forward-alias.sx
(x : A = 300 prints 300, not 44). 541/541 tests pass.

examples/0830-modules-flat-ns-same-name-forward-alias.sx

@@ -0,0 +1,16 @@
+// A flat import and a namespaced import both exporting `B :: u8` must not
+// preempt the own module's `A :: B; B :: u64;` forward alias chain.
+// Regression: issue 0107 — flat B :: u8 was winning before own B :: u64
+// was processed.
+
+std :: #import "modules/std.sx";
+#import "0830-modules-flat-ns-same-name-forward-alias/flat.sx";
+ns :: #import "0830-modules-flat-ns-same-name-forward-alias/ns.sx";
+A :: B;
+B :: u64;
+
+main :: () -> s32 {
+    x : A = 300;
+    std.print("{}\n", x);
+    0
+}

examples/0830-modules-flat-ns-same-name-forward-alias/flat.sx

@@ -0,0 +1 @@
+B :: u8;

examples/0830-modules-flat-ns-same-name-forward-alias/ns.sx

@@ -0,0 +1 @@
+B :: u8;

examples/expected/0830-modules-flat-ns-same-name-forward-alias.exit

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

examples/expected/0830-modules-flat-ns-same-name-forward-alias.stderr

@@ -0,0 +1 @@
+

examples/expected/0830-modules-flat-ns-same-name-forward-alias.stdout

@@ -0,0 +1 @@
+300