Fix: SwiftPM transitive linking for CNumKong via substantive shim source

[beshkenadze]

Refs #353. Supersedes #352.

Problem

CNumKong is declared as a regular SwiftPM target with no sources (just headers under include/numkong/ and include/module.modulemap). SwiftPM/Xcode 16+ fails at link time when this target is pulled transitively — it expects a CNumKong.o that's never produced (swiftlang/swift-package-manager#5706, open since October 2022).

Fix

Add a single C source file (include/cnumkong_module.c) that exports one real symbol, and declare it as the target's source. This produces a relocatable CNumKong.o that satisfies the linker without changing any public API.

+// CNumKong owns the public header umbrella and modulemap; the actual
+// runtime dispatch lives in CNumKongDispatch. The single `cnumkong_module.c`
+// source provides one real exported symbol so SwiftPM/Xcode 16+ can link
+// this target transitively (see swiftlang/swift-package-manager#5706).
+// Mirrors apple/swift-system's `Sources/CSystem/shims.c` pattern.
 .target(
     name: "CNumKong",
     path: "include",
+    sources: ["cnumkong_module.c"],
     publicHeadersPath: ".",
     ...
 ),

New file include/cnumkong_module.c:

// SwiftPM module marker for CNumKong.
//
// CNumKong is the header-only module facade for NumKong's public C API; the
// actual runtime dispatch implementation lives in the CNumKongDispatch target.
//
// SwiftPM/Xcode 16+ refuses to link a C target that produces no object files
// when it appears as a transitive dependency of an app target — the link step
// expects a CNumKong.o output that would otherwise never exist
// (see swiftlang/swift-package-manager#5706, open since October 2022).
//
// This file provides one real exported symbol so the relocatable link
// succeeds. The same pattern is used by apple/swift-system's
// Sources/CSystem/shims.c. There is no runtime cost — the symbol is read once
// at most by tooling that needs to verify the module is present.
#include "numkong/numkong.h"

const unsigned int nk_cnumkong_module_loaded = 1u;

Why this and not the alternatives

Approach	Result
Merge CNumKong into CNumKongDispatch	Cleanest structurally, but changes paths used by CMake / build.rs / setup.py. Larger blast radius — happy to send if preferred (see #353).
Substantive shim source (this PR)	Smallest diff. One new file with one symbol + comment. Same pattern as apple/swift-system. No behavioural change.
_dummy.c (rejected #352)	Static unused variable, no exported symbol, called «ad-hoc» in review. This PR's shim is the principled version of that idea.

Effect

• CNumKongDispatch and downstream consumers (USearch, etc.) keep the same import surface — the public header layout doesn't change.
• SwiftPM produces CNumKong.o with one symbol → transitive linking works in Xcode 16+.
• No runtime cost; nk_cnumkong_module_loaded is a single 32-bit constant.

Verification

• swift build and swift test pass in NumKong (standalone).
• swift build passes in a downstream macOS app (Tish) that depends on USearch via SwiftPM, transitively pulling CNumKongDispatch → CNumKong. Without this fix, the link step fails on missing CNumKong.o.
• No CMake / build.rs / setup.py paths touched.

Discussion

See #353 for the design walkthrough — including the merge alternative.