Profile cold unit module loading

site/doc/pages/request_perf.txt

@@ -15,6 +15,8 @@ Component resolution is divided into `component_path_us`, `component_artifact_us
 
 Successful first loads within the request are counted by `unit_load_count` and divided into `unit_module_us`, `unit_allocate_us`, `unit_import_us`, `unit_instantiate_us`, and `unit_initialize_us`. These cover compiled-module lookup, guest memory/table allocation, import construction, Wasmtime instantiation, and relocations/constructors/request binding. Repeated handlers from an already loaded unit are excluded.
 
+`unit_module_cache_hit_count` and `unit_module_cache_miss_count` divide module loads by the worker's compiled-module cache. A miss is further identified by `unit_module_serialized_cache_hit_count` when Wasmtime deserializes the current `.cwasm`; `unit_module_compile_count` means it fell back to compiling the `.wasm`. `unit_module_lookup_us`, `unit_module_read_us`, `unit_module_parse_us`, `unit_module_compile_us`, and `unit_module_classify_us` divide `unit_module_us` into artifact stat/cache lookup, wasm read, custom-section parse, deserialize-or-compile, and immutable import classification. The phase sum can be below the total because allocation and cache publication overhead remain in the aggregate.
+
 `unit_symbol_resolve_count` and `unit_symbol_resolve_us` isolate function and data symbol lookup within `unit_import_us`. The remainder of import time covers import-vector construction, Wasmtime Globals, GOT function table placement, and related bindings.
 
 Wasm FastCGI workers retain up to `MYSQL_PERSISTENT_POOL_SIZE` credential-keyed MySQL connections (default `8`; set `0` to disable). UCE calls the client library's connection-reset operation before another request receives a cached connection, clearing transactions, temporary tables, session variables, and selected databases while avoiding a new authentication handshake. Same-request leases continue to share state until request cleanup.