feat: Phase 9 — developer experience, extensibility, and community growth

New crates:
- quicproquo-bot: Bot SDK with polling API + JSON pipe mode
- quicproquo-kt: Key Transparency Merkle log (RFC 9162 subset)
- quicproquo-plugin-api: no_std C-compatible plugin vtable API
- quicproquo-gen: scaffolding tool (qpq-gen plugin/bot/rpc/hook)

Server features:
- ServerHooks trait wired into all RPC handlers (enqueue, fetch, auth,
  channel, registration) with plugin rejection support
- Dynamic plugin loader (libloading) with --plugin-dir config
- Delivery proof canary tokens (Ed25519 server signatures on enqueue)
- Key Transparency Merkle log with inclusion proofs on resolveUser

Core library:
- Safety numbers (60-digit HMAC-SHA256 key verification codes)
- Verifiable transcript archive (CBOR + ChaCha20-Poly1305 + hash chain)
- Delivery proof verification utility
- Criterion benchmarks (hybrid KEM, MLS, identity, sealed sender, padding)

Client:
- /verify REPL command for out-of-band key verification
- Full-screen TUI via Ratatui (feature-gated --features tui)
- qpq export / qpq export-verify CLI subcommands
- KT inclusion proof verification on user resolution

Also: ROADMAP Phase 9 added, bot SDK docs, server hooks docs,
crate-responsibilities updated, example plugins (rate_limit, logging).

crates/quicproquo-server/src/config.rs

@@ -31,6 +31,8 @@ pub struct FileConfig {
     #[serde(default)]
     pub metrics_enabled: Option<bool>,
     pub federation: Option<FederationFileConfig>,
+    /// Directory containing plugin `.so` / `.dylib` files to load at startup.
+    pub plugin_dir: Option<PathBuf>,
 }
 
 #[derive(Debug)]
@@ -51,6 +53,8 @@ pub struct EffectiveConfig {
     /// Start metrics server only when true and metrics_listen is set.
     pub metrics_enabled: bool,
     pub federation: Option<EffectiveFederationConfig>,
+    /// Directory to scan for plugin `.so` / `.dylib` files at startup. None = no plugins.
+    pub plugin_dir: Option<PathBuf>,
 }
 
 #[derive(Debug, Default, Deserialize)]
@@ -214,6 +218,8 @@ pub fn merge_config(args: &crate::Args, file: &FileConfig) -> EffectiveConfig {
         }
     };
 
+    let plugin_dir = args.plugin_dir.clone().or_else(|| file.plugin_dir.clone());
+
     EffectiveConfig {
         listen,
         data_dir,
@@ -228,6 +234,7 @@ pub fn merge_config(args: &crate::Args, file: &FileConfig) -> EffectiveConfig {
         metrics_listen,
         metrics_enabled,
         federation,
+        plugin_dir,
     }
 }