imports: dedup flat decl list by node identity (issue 0056 FIXED)

Impl blocks are anonymous (no declName), so a parameterised-protocol impl in a module reached via a diamond import was appended once per path and registered twice — 'duplicate impl Into for source s64'. mergeFlat and the directory-import merge loop now also dedup by node pointer; a physical AST node is lowered once regardless of how many import paths reach it.

Regression: examples/issue-0056-diamond-param-impl.sx.

examples/issue-0056/mid_a.sx

@@ -0,0 +1,3 @@
+#import "common.sx";
+
+mid_a_marker :: () -> s64 { 1; }