Reuse Android compiler outputs and remove old build artifacts

README.md

@@ -19,7 +19,9 @@ Supported options are `--local`, `--cpu {aarch64,arm,x86_64}`, `--no-fetch`, and
 
 Each build is one permanent Castle application transaction. Read `out/current/build.json` for its finalized state. On `PASS`, the current package is described by `out/current/package.json` and is at `out/current/kam-mobile.apk`.
 
-`build/<build_id>/` is transient working state. Its useful logs are `build/<build_id>/logs/compile.log`, `build/<build_id>/logs/package.log`, and, when apksigner is available and invoked, `build/<build_id>/logs/signature.log`.
+`build/work/<cpu>/application/` is the persistent compiler workspace. FPC reuses its compiled units without `-B`; source edits trigger incremental compilation. Changes to the manifest, source selection, or toolchain reset this workspace. Delete `build/work/` to force a clean build. Concurrent runner invocations are serialized.
+
+Each attempt keeps separate logs under `build/<build_id>/logs/`: `compile.log`, `package.log`, `compiler-cache.json` (cache reuse/reset details), and, when apksigner is available and invoked, `signature.log`.
 After every completed build, the compact logs and final `build.json` are copied to the tracked `build-evidence/<build_id>/` directory. It never contains the temporary application, compiler cache, or APK.
 
 ## Source and resources
@@ -55,7 +57,7 @@ config/          source and runtime-resource owners
 tools/           build, source-resolution, resource-staging, and device tools
 private-assets/  private runtime-resource contents
 vendor/          pinned vendor source
-build/           generated transient working state
+build/           persistent compiler workspace and per-attempt logs
 out/             generated current finalized state
 reports/         frozen Task-1 history
 dist/            frozen Task-1 history