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.

issues/0056-param-impl-not-deduped-across-diamond-import.md

@@ -0,0 +1,79 @@
+# 0056 — parameterised-protocol impl not deduped across a diamond import
+
+**FIXED** (`examples/issue-0056-diamond-param-impl.sx`, helpers in
+`examples/issue-0056/`). The flat decl list in
+[src/imports.zig](../src/imports.zig) now dedups by **node identity** as well
+as by name. `ResolvedModule.mergeFlat` and the directory-import merge loop each
+carry a `seen_nodes: std.AutoHashMap(*Node, void)` alongside the existing
+`seen_in_list` name set, and skip a decl whose pointer was already appended.
+
+## Symptom
+
+A module containing a parameterised-protocol impl (`impl Into(T) for S`) could
+not be imported through more than one path. Under a diamond —
+
+```
+main ─┬─ mid_a ─┐
+      └─ mid_b ─┴─ common   (holds `impl Into(Wrapped) for s64`)
+```
+
+— compilation failed with:
+
+```
+error: duplicate impl 'Into' for source 's64' in .../common.sx
+```
+
+This bit the moment `modules/std/objc.sx` (imported by `main.sx`,
+`platform/uikit.sx`, and `gpu/metal.sx` — a diamond) gained an
+`impl Into(*NSString) for string`.
+
+## Root cause
+
+`mergeFlat`/`addOwnDecl` dedup the global flat decl list by
+`decl.data.declName()`. Named decls (structs, fns, foreign classes) dedup
+fine across diamonds. But `impl_block` is anonymous — `declName()` returns
+`null` (see [src/ast.zig](../src/ast.zig) `Data.declName`) — so the dedup
+guard was skipped and the **same cached** impl node (modules are cached in
+`ModuleCache`, so both paths share one node pointer) was appended once per
+path. `registerParamImpl` in [src/ir/lower.zig](../src/ir/lower.zig) then saw
+two entries with the same `defining_module` and raised the same-file
+"duplicate impl" diagnostic.
+
+The pre-existing `impl Into(Block) for Closure(...)` in
+`modules/std/objc_block.sx` never tripped this because that module is not
+imported through any diamond.
+
+## Reproduction
+
+`examples/issue-0056/common.sx`:
+
+```sx
+Wrapped :: struct { v: s64; }
+
+impl Into(Wrapped) for s64 {
+    convert :: (self: s64) -> Wrapped {
+        return .{ v = self };
+    }
+}
+```
+
+`mid_a.sx` / `mid_b.sx` each `#import "common.sx";`. The diamond main:
+
+```sx
+#import "modules/std.sx";
+#import "issue-0056/mid_a.sx";
+#import "issue-0056/mid_b.sx";
+
+main :: () -> s32 {
+    w : Wrapped = xx 7;   // pre-fix: duplicate impl 'Into' for source 's64'
+    print("{}\n", w.v);   // post-fix: prints 7
+    0;
+}
+```
+
+## Fix
+
+Dedup the flat decl list by node identity in addition to name — a physical
+AST node must be lowered once regardless of how many import paths reach it.
+Named-decl first-wins behaviour is unchanged. Regression test:
+`examples/issue-0056-diamond-param-impl.sx` (in `tests/run_examples.sh`).