[M1] add layout::discover

CHANGELOG.md

@@ -29,3 +29,10 @@ All notable changes to cargoxx will be documented in this file.
   Dependencies are emitted alphabetically (matches Cargo). Round-trip
   property is exercised by `tests/manifest_write.cpp` (9 cases).
   Defaulted `operator==` on the manifest structs supports comparison.
+- `cargoxx.layout` public types (`Target`, `TargetKind`, `DiscoveredLayout`)
+  and `discover(project_root, package_name)`. Walks `src/` recursively
+  for the library (excluding the `src/bin/` subtree), enumerates
+  `src/main.cpp`, `src/bin/*.cpp`, `tests/*.cpp`, `examples/*.cpp` flat,
+  sorts results for deterministic output, and returns `LayoutNoTarget`
+  when neither a library nor any binary is present.
+  `tests/layout_discovery.cpp` covers 12 cases.