feat: add multi-engine regex flavour support

docs/PERFORMANCE.md

@@ -19,12 +19,23 @@ supervisor wall clock, and cancellation discards the complete worker heap.
 Python has a materially larger cold start because its worker loads the
 self-hosted Pyodide JavaScript/WebAssembly runtime and Python standard library
 before verifying CPython and `re`. Java imports and self-tests its bundled
-TeaVM ES module. Engine loading is an explicit operation with a
-flavour-specific startup deadline; it is not charged to the 250 ms live or
-selected manual execution timeout. Warm requests reuse the verified worker.
-Killing a worker for timeout, crash or cancellation discards its complete
-runtime, so the next request pays the cold-load cost and verifies identity
-again.
+TeaVM ES module. Scala compatibility shares that Java module and does not load
+a second runtime.
+
+PHP, legacy Perl, Ruby, Go, Rust, C++ and .NET likewise load only when first
+selected. Their cold cost differs substantially: PHP, Perl, Ruby and .NET
+instantiate larger language/runtime packs; Go includes its standard
+WebAssembly runtime; Rust and C++ are comparatively compact native modules.
+The UI reports loading separately rather than charging it to the 250 ms live or
+selected manual execution timeout.
+
+Warm requests reuse a verified worker. To prevent profile exploration from
+leaving every large runtime resident, the supervisor uses a two-worker
+retention target and evicts the least recently used idle generation before it
+creates another profile worker. An active request is never evicted, so
+concurrent active work can temporarily exceed the target. Killing or evicting
+a worker discards its complete runtime, so the next request pays the cold-load
+cost and verifies identity again.
 
 Automatic-callout tracing is deliberately excluded from every benchmark. It
 compiles an independently instrumented PCRE2 pattern in a separate worker and