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/common.sx

@@ -0,0 +1,12 @@
+// A parameterised-protocol (`Into`) impl living in a module reachable through
+// more than one import path. Each path must NOT re-register the impl.
+// Impl blocks are anonymous (`declName() == null`), so the diamond-import
+// dedup in imports.zig (`mergeFlat`) skips them and appends the node once per
+// path — `registerParamImpl` then trips its same-file duplicate check.
+Wrapped :: struct { v: s64; }
+
+impl Into(Wrapped) for s64 {
+    convert :: (self: s64) -> Wrapped {
+        return .{ v = self };
+    }
+}

examples/issue-0056/mid_a.sx

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

examples/issue-0056/mid_b.sx

@@ -0,0 +1,3 @@
+#import "common.sx";
+
+mid_b_marker :: () -> s64 { 2; }