fix: key the host PCM cache on source content, add clean, hash project args

Three follow-ons to the stale-build report, all cases of an identity not
capturing something that changes the output.

The host PCM cache under <cache>/crafter.build/<target>-<march>/ is shared by
every crafter-build on the machine, and freshness was a per-file mtime
comparison. That cannot tell "this PCM is newer than my source" from "this PCM
was built from different sources that happen to be newer", so a package install
and a working checkout — or two checkouts of different versions — silently
compiled their project.cpp against each other's declarations. Invalidation now
keys on a stamp over the bytes of every module source, which also covers the
case one file's mtime never could: the cached PCMs import each other, so a
change to :Interface invalidates :Clang's PCM with Crafter.Build-Clang.cppm
untouched.

Project args ApplyStandardArgs does not itself interpret are now folded into
VariantId. Such a flag typically decides what gets compiled or bundled — the
report's example is --no-webgpu dropping entries from cfg.files — and without it
both settings shared one bin dir and interleaved their outputs there, leaving a
bundle matching neither. Sorted and deduplicated so flag order doesn't split the
cache, and inherited by test Configurations.

`crafter-build clean` removes the project's bin/ and build/ trees. It
deliberately does not load project.cpp: cleaning is most often reached when
something is already wrong, and a clean that first needs the project to compile
is useless exactly then.

interfaces/Crafter.Build-Clang.cppm

@@ -250,6 +250,15 @@ export namespace Crafter {
         // clobber the baseline's. Not for direct project use — declare
         // wasmVariants instead.
         std::vector<std::string> wasmVariantFlags;
+        // The project args ApplyStandardArgs did not interpret — i.e. the
+        // project's own flags, whose effect on the build the framework cannot
+        // see. Hashed into VariantId because such a flag typically decides what
+        // gets compiled or bundled (`--no-webgpu` dropping entries from
+        // cfg.files, say), and without it both flag settings share one bin dir
+        // and interleave their outputs there. Populated by ApplyStandardArgs;
+        // the args it does recognise are excluded, since their effect already
+        // shows up in target/march/mtune/debug/sysroot/type.
+        std::vector<std::string> projectArgs;
         std::vector<Test> tests;
         // Lint rules for `crafter-build lint`. Populate via AddLintRule.
         std::vector<LintRule> lintRules;
@@ -315,6 +324,12 @@ export namespace Crafter {
                 compileKey += "|W:";
                 compileKey += f;
             }
+            // Sorted by ApplyStandardArgs, so flag order on the command line
+            // doesn't split the cache.
+            for (const std::string& a : projectArgs) {
+                compileKey += "|A:";
+                compileKey += a;
+            }
             std::size_t configHash = std::hash<std::string>{}(compileKey);
             return std::format("{}-{}-{}-{}-{:08x}", name, target, march, mtune, configHash);
         }
@@ -345,6 +360,16 @@ export namespace Crafter {
 
     CRAFTER_API int Run(int argc, char** argv);
 
+    // Delete the bin/ and build/ trees beside `projectFile`, returning the paths
+    // that existed and were removed. Backs `crafter-build clean`.
+    //
+    // Deliberately does not load the project: cleaning is most often reached
+    // when something is already wrong, and a clean that first needs project.cpp
+    // to compile (and its git dependencies to be present) is useless exactly
+    // then. That means variant directories are not enumerated — the whole tree
+    // goes, which is what the by-hand `rm -rf bin build` did anyway.
+    CRAFTER_API std::vector<fs::path> CleanProject(const fs::path& projectFile);
+
     // Add a small index.html + runtime.js pair next to the .wasm output so the
     // build can be loaded directly in a browser (just `serve` the bin dir and
     // open it). Opt-in: WASI builds destined for wasmtime/wasmer don't need