fix(C4): collectCImportSources recurses into nested namespaces

A named #import c unit declared inside an aliased module sits two
namespace levels deep in the merged tree; the one-level walk (the
extractLibraries/0130 pattern in c_import form) never collected it,
so the unit silently never compiled and its symbols resolved from
whatever process image carried the same names — surfaced by C4's
sqlite migration, where only the version pin could tell the OS copy
from the vendored one.

examples/1623-cimport-unit-in-aliased-module.sx

@@ -0,0 +1,11 @@
+// A named `#import c` unit declared INSIDE an aliased module (so the
+// unit's namespace nests two deep in the merged tree) still compiles
+// and links — collectCImportSources recurses through namespaces, the
+// same way #library collection does (issue 0130's pattern).
+#import "modules/std.sx";
+m :: #import "1623-cimport-unit-in-aliased-module/mod.sx";
+
+main :: () -> i32 {
+    print("nested unit answer = {}\n", m.answer_via_mod());
+    0
+}

examples/1623-cimport-unit-in-aliased-module/inmod.c

@@ -0,0 +1 @@
+int unit_in_mod_answer(void) { return 54; }

examples/1623-cimport-unit-in-aliased-module/mod.sx

@@ -0,0 +1,10 @@
+// A module that owns a named C unit + a hand-curated binding.
+#import "modules/std.sx";
+
+cunit :: #import c {
+    #source "inmod.c";
+};
+
+unit_in_mod_answer :: () -> i32 #foreign cunit "unit_in_mod_answer";
+
+answer_via_mod :: () -> i32 { return unit_in_mod_answer(); }

examples/expected/1623-cimport-unit-in-aliased-module.exit

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

examples/expected/1623-cimport-unit-in-aliased-module.stdout

@@ -0,0 +1 @@
+nested unit answer = 54