feat: add ZRTL effects plugin for algebraic effects runtime

Implements runtime support for algebraic effects:
- Thread-local handler stack with effect ID cache
- Handler management: push_handler, pop_handler, get_handler
- Effect dispatch: perform (runtime lookup), perform_direct (static)
- State management: alloc_state, free_state, get_state
- Effect ID generation via FNV-1a hash

Exported symbols:
- $Effects$push_handler, $Effects$pop_handler, $Effects$get_handler
- $Effects$has_handler, $Effects$handler_depth, $Effects$get_state
- $Effects$perform, $Effects$perform_direct
- $Effects$alloc_state, $Effects$free_state
- $Effects$id_from_name, $Effects$debug_stack, $Effects$clear_all

All 4 tests pass.

Author: darmie

plugins/Cargo.toml

@@ -29,6 +29,7 @@ members = [
     "zrtl_audio",
     "zrtl_text",
     "zrtl_model",
+    "zrtl_effects",
 ]
 
 [workspace.dependencies]

plugins/zrtl_effects/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "zrtl_effects"
+version = "0.1.0"
+edition = "2021"
+description = "ZRTL Effects plugin - Algebraic effects runtime support"
+license = "Apache-2.0"
+
+[lib]
+crate-type = ["cdylib", "staticlib", "rlib"]
+
+[dependencies]
+zrtl = { path = "../../sdk/zrtl" }
+libc = "0.2"