fix: track what a primary module interface imports

A primary module interface unit — `export module Widget;`, no partitions —
recorded nothing about what it imported. GetInterfacesAndImplementations
registered the Module and then erased the file from the scan list, so the
import pass only ever saw partitions, and Module had no vectors to hold an
edge anyway.

Two consequences, both reported as issue #26:

  Module::Check consulted only its own .cppm and its partitions. A data
  member added to an imported module left Widget.pcm, Widget.o and every
  consumer object untouched while the imported library rebuilt and both
  binaries relinked — one executable holding two class layouts, no
  diagnostic, and a crash somewhere unrelated. Wiping build/ was the only
  cure, so `crafter-build test` could not be trusted straight after an
  interface edit.

  Module::Compile waited on nothing. Two modules in one Configuration
  compile on concurrent threads, so a primary interface importing a
  sibling was a coin flip between working and "module 'Base' not found".

Partitions never had either problem — they carry the same three vectors and
Check/Compile honour them — which is why the gap only surfaced on a module
whose interface is one flat unit.

Module now carries moduleDependencies, externalModuleDependencies and
pendingImports with the same meanings as on ModulePartition; primary units
stay in the scan list so their imports land there; Check sees through them;
Compile orders itself behind a local sibling; and ResolvePendingImports
sweeps them so an edge survives dependencies being wired up afterwards.
Build() now Checks every interface before spawning any compile thread — the
`compiled` flag a waiter blocks on is raised either by a Compile that runs
or by the Check that decides none is needed, so a Check still pending while
another module's thread waits would have hung the build.

Resolves #26

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

implementations/Crafter.Build-Interface.cpp

@@ -39,6 +39,10 @@ namespace Crafter {
                 }
                 needsRecompiling = false;
                 compiled.store(true);
+                // Nothing waits on `compiled` until the compile threads start,
+                // which is after every Check has run — but notify anyway so the
+                // flag is never left set without a wake-up behind it.
+                compiled.notify_all();
                 return false;
             } else {
                 needsRecompiling = true;
@@ -100,24 +104,55 @@ namespace Crafter {
             std::string pcmPath = std::format("{}.pcm", (pcmDir/path.filename()).generic_string());
             std::string cppmPath = std::format("{}.cppm", path.generic_string());
             if(fs::exists(pcmPath) && std::max(fs::last_write_time(cppmPath), sourceFloor) < fs::last_write_time(pcmPath)) {
+                fs::file_time_type pcmTime = fs::last_write_time(pcmPath);
+                // Every partition gets Check()ed even once one is known stale:
+                // Compile() drives partition rebuilds off their own
+                // needsRecompiling flags, so short-circuiting here would leave
+                // the later ones unevaluated and silently unbuilt.
                 bool depCheck = false;
                 for(std::unique_ptr<ModulePartition>& partition : partitions) {
                     if(partition->Check(pcmDir, sourceFloor)) {
                         depCheck = true;
                     }
                 }
+                // Modules this interface unit imports directly. Local ones share
+                // our pcmDir and are resolved recursively; external ones are
+                // compared by PCM mtime, their owning Configuration having
+                // already finished building by the time we run.
+                for(Module* dependency : moduleDependencies) {
+                    if(dependency->Check(pcmDir, sourceFloor)) {
+                        depCheck = true;
+                    }
+                }
+                if(!depCheck) {
+                    for(const auto& [externalMod, externalPcmPath] : externalModuleDependencies) {
+                        std::error_code ec;
+                        fs::file_time_type t = fs::last_write_time(externalPcmPath, ec);
+                        if (!ec && t >= pcmTime) {
+                            depCheck = true;
+                            break;
+                        }
+                    }
+                }
                 if(depCheck) {
                     needsRecompiling = true;
                     return true;
                 } else {
                     needsRecompiling = false;
                     compiled.store(true);
+                    compiled.notify_all();
                     return false;
                 }
             } else {
+                // Already known stale, but the dependencies still need
+                // evaluating: an unchecked local module would never get built,
+                // and Compile() below waits on its `compiled` flag.
                 for(std::unique_ptr<ModulePartition>& partition : partitions) {
                     partition->Check(pcmDir, sourceFloor);
                 }
+                for(Module* dependency : moduleDependencies) {
+                    dependency->Check(pcmDir, sourceFloor);
+                }
                 needsRecompiling = true;
                 return true;
             }
@@ -127,6 +162,17 @@ namespace Crafter {
     }
 
     void Module::Compile(const std::string_view clang, const fs::path& pcmDir, const fs::path& buildDir, std::atomic<bool>& buildCancelled, std::string& buildError) {
+        // A sibling module in the same Configuration that this interface unit
+        // imports has to have its PCM on disk before we precompile against it.
+        // Safe to block here: Build() Checks every interface before it spawns
+        // any compile thread, so a dependency that needs no rebuild already has
+        // `compiled` set and this returns immediately.
+        for(Module* dependency : moduleDependencies) {
+            if(!dependency->compiled.load()) {
+                dependency->compiled.wait(false);
+            }
+        }
+
         std::vector<std::thread> threads;
         threads.reserve(partitions.size());
         for(std::unique_ptr<ModulePartition>& part : partitions) {