feat: plugin-declared parallel execution, switch to env_logger, update deps

Parallel execution (opt-in via MetaPlugin::parallel_safe):
- Add Send bound to MetaPlugin, parallel_safe() method (default false)
- Override to true in digest, tokens, exec, magic_file plugins
- MetaService: std::thread::scope for initialize_plugins and process_chunk
- Extract plugins via NullMetaPlugin sentinel + std::mem::replace (no unsafe)
- Panic tracking: join errors logged, NullMetaPlugin restored and finalized
- MetaPluginExec: Box<dyn Write> -> Box<dyn Write + Send>
- SendCookie wrapper for libmagic Cookie with unsafe impl Send

Logging (stderrlog -> env_logger):
- Custom format: [SSSSSS.mmm] LEVEL [module:] message (time-since-start ms)
- Default level: Warn (matches previous behavior)
- -v: Debug, -vv+: Trace, -q: off
- -vv+ shows module path

Maintenance:
- Bump deps: thiserror 2.0, config 0.15, dns-lookup 3.0, lz4_flex 0.12,
  ringbuf 0.4, rand 0.9, lazy_static 1.5, env_logger 0.11
- Update Cargo.lock (186 transitive packages)
- Clippy fixes: is_multiple_of, to_string_in_format_args, collapsible_if
- Fix double-counting bug in TokensMetaPlugin::update
- Fix schema description using plugin.description()

Co-Authored-By: opencode <noreply@opencode.ai>

src/meta_plugin/exec.rs

@@ -20,7 +20,7 @@ pub struct MetaPluginExec {
     pub supported: bool,
     pub split_whitespace: bool,
     process: Option<Child>,
-    writer: Option<Box<dyn Write>>,
+    writer: Option<Box<dyn Write + Send>>,
     result: Option<String>,
     base: BaseMetaPlugin,
 }
@@ -263,6 +263,10 @@ impl MetaPlugin for MetaPluginExec {
     fn default_outputs(&self) -> Vec<String> {
         vec!["exec".to_string()]
     }
+
+    fn parallel_safe(&self) -> bool {
+        true
+    }
 }
 
 use crate::meta_plugin::register_meta_plugin;