fix(link): platform-aware dep archive name (Windows .lib, not hardcoded .a)

The dep static archive was named lib<pkg>.a on every platform; Windows uses
static_lib_ext=.lib / empty lib_prefix, so the hardcoded name broke the Windows
test link. Use platform::lib_prefix + static_lib_ext (mirrors target_output),
placed in bin/. e2e 07 (static lib) is requires:elf so this archive path was
never exercised on Windows before.

Author: Sunrisepeak

src/build/plan.cppm

@@ -418,7 +418,13 @@ BuildPlan make_plan(const mcpp::manifest::Manifest&         manifest,
         LinkUnit ar;
         ar.targetName = qname;
         ar.kind       = LinkUnit::StaticLibrary;
-        ar.output     = std::filesystem::path("lib" + sanitize(qname) + ".a");
+        // Platform-aware archive name (libfoo.a on ELF/Mach-O, foo.lib on
+        // Windows) — mirrors target_output() so the toolchain's `ar` + lld
+        // accept it on every platform. (Hardcoding `.a` broke Windows, whose
+        // static_lib_ext is `.lib`.)
+        ar.output     = std::filesystem::path("bin") /
+            std::format("{}{}{}", mcpp::platform::lib_prefix, sanitize(qname),
+                        mcpp::platform::static_lib_ext);
         ar.objects    = std::move(archiveObjs);
         staticDepPackages.insert(qname);
         staticDepArchives.push_back({qname, ar.output});