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).

examples/plugins/logging_plugin/Cargo.toml

@@ -0,0 +1,15 @@
+# This is a standalone cdylib crate outside the main workspace.
+[workspace]
+
+[package]
+name    = "logging_plugin"
+version = "0.1.0"
+edition = "2021"
+description = "Reference quicproquo server plugin: logs all hook events to stderr."
+license = "MIT"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+quicproquo-plugin-api = { path = "../../../crates/quicproquo-plugin-api" }