Keep precompile PCH generation-local

docs/wasi-sdk-toolchain.md

@@ -39,6 +39,12 @@ UCE expects these executables on each deployment host:
 source map before debug sections are stripped from the runtime artifact. The
 map is consulted only on a wasm trap.
 
+The native compiler passes the ABI-generation root to
+`scripts/compile_wasm_unit`. Unless `UCE_WASM_PCH_DIR` is explicitly set, the
+unit PCH lives beneath that writable generation rather than a
+process-user-dependent `/tmp` path. Managed precompile therefore behaves the
+same inside and outside the service unit.
+
 ## Upgrade policy
 
 Treat WASI SDK upgrades like runtime dependency upgrades:

docs/wasm-runtime-architecture.md

@@ -233,7 +233,7 @@ whose compilation outlasted the guest CPU budget would immediately trap in the
 following allocator/relocation call even though no guest loop consumed it.
 
 Unit artifacts live beneath an ABI-generation directory such as
-`BIN_DIRECTORY/units-c12-w7`: `c12` is the compiler/unit-metadata ABI and `w7`
+`BIN_DIRECTORY/units-c13-w7`: `c13` is the compiler/unit-metadata ABI and `w7`
 is the runtime core ABI. Old and new service binaries therefore never publish
 or read the same unit path during an ABI transition. The managed restart builds
 and serializes the complete next generation before stopping the old service;