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/mod.rs

@@ -318,7 +318,7 @@ pub fn process_metadata_outputs(
     })
 }
 
-pub trait MetaPlugin
+pub trait MetaPlugin: Send
 where
     Self: 'static,
 {
@@ -488,6 +488,17 @@ where
         ""
     }
 
+    /// Returns true if this plugin can execute concurrently with other
+    /// parallel-safe plugins.
+    ///
+    /// Plugins that do significant per-chunk work (hashing, tokenization,
+    /// piping to child processes) should return true. The MetaService will
+    /// run all parallel-safe plugins in separate threads per phase, then
+    /// process results sequentially.
+    fn parallel_safe(&self) -> bool {
+        false
+    }
+
     /// Builds the schema for this plugin from its options and outputs.
     ///
     /// Default implementation infers option types from YAML values and