feat: add plugin schema system, tokenizer cache, and config validation

- Add plugin schema types and runtime discovery for meta/filter plugins
- Rewrite --generate-config to use schema system instead of hardcoded types
- Add Settings::validate_config() for startup validation
- Cache tokenizer instances via static Lazy to avoid repeated BPE loading
- Add split_by_token_iter() and count_bounded() to Tokenizer
- Fix double-counting bug in TokensMetaPlugin when buffer < max_buffer_size
- Eliminate unnecessary allocations in token count methods
- Refactor token filters: remove Option<Tokenizer>, use iterator API
- Fix TailTokensFilter correctness: unbounded buffer instead of ring buffer
- Add encoding option to all token filters
- Add description() to MetaPlugin and FilterPlugin traits
- Fix unused_mut warning in compression engine (feature-gated code)

Co-Authored-By: code-review-bot <noreply@anthropic.com>

src/config.rs

@@ -573,4 +573,65 @@ impl Settings {
             .map(|plugins| plugins.iter().map(|p| p.name.clone()).collect())
             .unwrap_or_default()
     }
+
+    /// Validates the configuration against plugin schemas.
+    ///
+    /// Checks that:
+    /// - All configured meta plugin names are valid and registered
+    /// - Required options are present for each meta plugin
+    /// - Compression plugin name (if set) is a valid compression type
+    ///
+    /// Returns a list of warning strings. An empty list means the config is valid.
+    pub fn validate_config(&self) -> Vec<String> {
+        use crate::common::schema::gather_meta_plugin_schemas;
+        use crate::compression_engine::CompressionType;
+        use strum::IntoEnumIterator;
+
+        let mut warnings = Vec::new();
+
+        // Validate compression plugin
+        if let Some(ref comp) = self.compression_plugin {
+            let valid_types: Vec<String> =
+                CompressionType::iter().map(|ct| ct.to_string()).collect();
+            if !valid_types.contains(&comp.name) {
+                warnings.push(format!(
+                    "Unknown compression_plugin.name: '{}'. Valid types: {}",
+                    comp.name,
+                    valid_types.join(", ")
+                ));
+            }
+        }
+
+        // Validate meta plugins
+        if let Some(ref plugins) = self.meta_plugins {
+            let schemas = gather_meta_plugin_schemas();
+            let schema_map: std::collections::HashMap<&str, &crate::common::schema::PluginSchema> =
+                schemas.iter().map(|s| (s.name.as_str(), s)).collect();
+
+            for plugin in plugins {
+                match schema_map.get(plugin.name.as_str()) {
+                    Some(schema) => {
+                        // Check required options
+                        for opt in &schema.options {
+                            if opt.required && !plugin.options.contains_key(&opt.name) {
+                                warnings.push(format!(
+                                    "Meta plugin '{}': missing required option '{}'",
+                                    plugin.name, opt.name
+                                ));
+                            }
+                        }
+                    }
+                    None => {
+                        warnings.push(format!(
+                            "Unknown meta plugin: '{}'. Available: {}",
+                            plugin.name,
+                            schema_map.keys().copied().collect::<Vec<_>>().join(", ")
+                        ));
+                    }
+                }
+            }
+        }
+
+        warnings
+    }
 }