Parallelize proactive unit compilation

docs/setup.md

@@ -190,6 +190,7 @@ SITE_DIRECTORY=/var/www/html
 HTTP_DOCUMENT_ROOT=/var/www/html
 JIT_COMPILE_ON_REQUEST=1
 PROACTIVE_COMPILE_ENABLED=1
+PROACTIVE_COMPILE_JOBS=2
 PROACTIVE_COMPILE_CHECK_INTERVAL=60
 
 WASM_COMPILE_SCRIPT=scripts/compile_wasm_unit
@@ -224,6 +225,7 @@ Important settings:
 - `HTTP_PORT` is the built-in HTTP/WebSocket listener used for WebSocket upgrade traffic and direct local probes. Bind/firewall it for local access only; nginx/Apache should be the public entry point.
 - `WS_BROKER_OUTBOUND_TIMEOUT_SECONDS` controls how long a forwarded WS message can remain queued in the broker before being dropped (default `30`). Set to `0` to disable the timeout.
 - `WASM_COMPILE_SCRIPT` must point to `scripts/compile_wasm_unit` unless you provide an equivalent compiler. Relative paths are resolved from the runtime root/`COMPILER_SYS_PATH`. That script calls `scripts/check_unit_wasm.py` after linking each unit and uses the pinned WASI SDK on every deployment host.
+- `PROACTIVE_COMPILE_JOBS` selects 1–16 low-priority full-site scanner processes (default `2`). Each canonical unit path has one scanner owner. The separate higher-priority demand compiler remains reserved for stale units requested over HTTP, so total background compile concurrency can reach this value plus one.
 - `WASM_CORE_PATH` must point at the built `core.wasm` file.
 
 After editing settings, restart UCE: