feat(incremental): rebuild when an #included header changes

The staleness check compared an artifact against its own source and its
module imports. Headers were in neither set: the scanner reads `import`
lines, and a .cppm or .cpp keeps its mtime when a header it includes is
edited — so the build reported nothing to do and left objects compiled
against the previous contents. Same silent mixed-layout binary as issue
27, reached through #include instead of import.

Ask the compiler what it actually opened. Every C++ and C compile now
passes -MD -MF <artifact>.d, and Check reads that depfile back through
NewestPrerequisite, comparing every prerequisite's mtime against the
artifact. A missing depfile (an object from a crafter-build that wrote
none) or a prerequisite that no longer exists reads as "rebuild": neither
is evidence of freshness.

The parser unescapes make syntax rather than splitting on whitespace,
since clang wraps depfiles onto continuation lines and escapes spaces in
filenames.

README.md

@@ -121,12 +121,15 @@ Per-import precise tracking for both within-project and cross-project module dep
 
 - Touch `lib/Hello.cppm` → only consumers of `Hello` rebuild.
 - Touch `lib/Other.cppm` → only consumers of `Other` rebuild.
+- Touch a header → only the units that `#include` it (directly or transitively) rebuild.
 - External CMake dep produces fresh `.a` files → whole project rebuilds (deliberately coarse — cmake-dep changes are rare).
 
 Diamond deps (`A → {B, C}; B → X; C → X`) build `X` exactly once via a `std::shared_future<BuildResult>` cache.
 
 Tracking is derived from each translation unit's `import` statements, which are scanned when the sources are declared. Every kind of unit is scanned — primary module interfaces, partitions and implementation units alike — and an interface that imports a sibling module in the same `Configuration` also gets its compile ordered behind it. `cfg.dependencies` is often assigned *afterwards* — `AddTest` works that way — so `Build()` re-resolves any import that matched nothing at scan time before it compares mtimes. Without all of that, a consumer of a dependency's module carried no edge to it at all: adding a data member to that dependency's interface rebuilt the library, relinked the consumer, and left the consumer's object compiled against the old class layout. Nothing fails to link when a member is added, so the result was a working build and a crash later.
 
+`#include`s are tracked separately, because scanning can't see them: a header pulls in more headers, and which ones are open depends on the preprocessor state at the point of inclusion. So every compile is asked to report what it actually opened — `clang -MD -MF <artifact>.d`, written next to the object or BMI it belongs to — and the next staleness check reads that file back and compares every prerequisite's mtime against the artifact. C sources are covered the same way, and there they are the *only* dependency record. An artifact whose depfile is missing, or that names a prerequisite which no longer exists, is rebuilt once: neither state is evidence of freshness. Before this, a header edit changed no `.cppm` or `.cpp` mtime at all, so a build reported nothing to do and left objects compiled against the old contents — the same mixed-layout failure as above, reached through `#include` instead of `import`.
+
 Everything that changes what gets built belongs in the variant id, since it names the `bin/` and `build/` directory. That includes project args crafter-build itself doesn't interpret: `crafter-build` and `crafter-build -- --no-webgpu` get separate directories rather than interleaving their outputs in one. The cached host PCMs under `<cache>/crafter.build/<target>-<march>/` are shared by every crafter-build on the machine, so they are invalidated by a hash of the module sources rather than by mtime — an mtime can't tell a newer PCM from one built by a different install.
 
 `crafter-build clean` removes the project's `bin/` and `build/` trees. It doesn't load `project.cpp`, so it still works when the project no longer compiles.