# 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`).