feat: duplicate C exports across #import c units are a compile diagnostic

All units share one link namespace (per-unit isolation is PLAN-C C3.2,
deferred), so a symbol defined by two units previously died inside the
JIT dylib link or the AOT link with raw linker spew. The clang shim
gains sx_clang_object_exported_symbols (llvm::object scan: defined +
global, format-specific excluded) and compileCToObjects cross-checks
every unit object — collisions name both source files. Scan failures
are non-fatal; the linker remains the backstop. Covers JIT and native
AOT; the emcc path still relies on wasm-ld's own error.

examples/1628-cimport-duplicate-export.sx

@@ -0,0 +1,20 @@
+// Two `#import c` units defining the same exported symbol is a
+// compile-time diagnostic naming both sources — previously it died
+// inside the JIT dylib link (or the AOT link) with raw linker spew.
+// All units share one link namespace; per-unit symbol isolation is
+// PLAN-C C3.2 (deferred).
+#import "modules/std.sx";
+
+ua :: #import c {
+    #source "1628-cimport-duplicate-export/a.c";
+};
+ub :: #import c {
+    #source "1628-cimport-duplicate-export/b.c";
+};
+
+clash :: () -> i32 #foreign ua "clash";
+
+main :: () -> i32 {
+    print("{}\n", clash());
+    0
+}

examples/1628-cimport-duplicate-export/a.c

@@ -0,0 +1 @@
+int clash(void) { return 1; }

examples/1628-cimport-duplicate-export/b.c

@@ -0,0 +1 @@
+int clash(void) { return 2; }

examples/expected/1628-cimport-duplicate-export.exit

@@ -0,0 +1 @@
+1

examples/expected/1628-cimport-duplicate-export.stderr

@@ -0,0 +1 @@
+error: C symbol 'clash' is defined by multiple '#import c' sources: 'examples/1628-cimport-duplicate-export/a.c' and 'examples/1628-cimport-duplicate-export/b.c' — all units share one link namespace