Add a module (plugin) system for bot features

Introduce a `Module` trait (src/bot/module.rs): a feature declares the
commands it provides and turns each invocation into a `Vec<Action>`, without
touching the connection. Modules are pure — context in, actions out — so they
are unit-testable in isolation and avoid borrowing the `Bot` that owns them.

Bot::new builds the module set and a command -> module route table; the
`commands` dispatcher routes to the owning module and executes its actions,
and `help` is auto-generated from every module's CommandSpec metadata.

The existing ping/echo/hello move into a `builtins` module, and a stateful
`dice` module (`roll [NdM]`, with its own dependency-free xorshift PRNG) is
added as a worked example. Each network gets its own module set, so stateful
modules keep independent per-network state with no locking. Adding a feature
is now a new file in bot/modules/ plus one line in modules::all().

Tests: new tests/modules.rs drives modules through the trait with no network;
end-to-end dispatch and auto-help verified against a fake server. No new deps.

src/bot/modules/dice.rs

@@ -0,0 +1,99 @@
+//! A dice roller: `roll 2d6`, `roll d20`, or `roll` (defaults to `1d6`).
+//!
+//! Demonstrates a *stateful* module — it carries its own tiny PRNG, seeded once
+//! at construction — while keeping the crate dependency-free (no `rand`).
+
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use crate::bot::module::{Action, Command, CommandSpec, Module};
+
+const COMMANDS: &[CommandSpec] = &[CommandSpec {
+    name: "roll",
+    usage: "roll [NdM]",
+    about: "roll N dice of M sides (default 1d6), e.g. roll 2d20",
+}];
+
+/// Upper bounds to keep output (and the channel) sane.
+const MAX_DICE: u64 = 100;
+const MAX_SIDES: u64 = 1000;
+
+/// Dice roller with a self-contained xorshift PRNG — fine for dice, not for
+/// anything security-sensitive.
+pub struct Dice {
+    rng: u64,
+}
+
+impl Dice {
+    pub fn new() -> Dice {
+        let seed = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .map(|d| d.as_nanos() as u64)
+            .unwrap_or(0x9E37_79B9_7F4A_7C15);
+        Dice { rng: seed | 1 } // never seed to 0 (xorshift would stay stuck)
+    }
+
+    fn next_u64(&mut self) -> u64 {
+        let mut x = self.rng;
+        x ^= x << 13;
+        x ^= x >> 7;
+        x ^= x << 17;
+        self.rng = x;
+        x
+    }
+
+    /// Roll a single die in `1..=sides`.
+    fn die(&mut self, sides: u64) -> u64 {
+        self.next_u64() % sides + 1
+    }
+}
+
+impl Default for Dice {
+    fn default() -> Dice {
+        Dice::new()
+    }
+}
+
+impl Module for Dice {
+    fn name(&self) -> &'static str {
+        "dice"
+    }
+
+    fn commands(&self) -> &'static [CommandSpec] {
+        COMMANDS
+    }
+
+    fn on_command(&mut self, cmd: &Command) -> Vec<Action> {
+        let spec = cmd.args.first().copied().unwrap_or("1d6");
+        let reply = match parse_spec(spec) {
+            Some((count, sides)) => {
+                let rolls: Vec<u64> = (0..count).map(|_| self.die(sides)).collect();
+                let total: u64 = rolls.iter().sum();
+                if count == 1 {
+                    format!("{count}d{sides}: {total}")
+                } else {
+                    let parts: Vec<String> = rolls.iter().map(u64::to_string).collect();
+                    format!("{count}d{sides}: {} = {total}", parts.join(" + "))
+                }
+            }
+            None => format!("usage: {p}roll [NdM], e.g. {p}roll 2d6", p = cmd.prefix),
+        };
+        vec![Action::Reply(reply)]
+    }
+}
+
+/// Parse `NdM` / `dM` / `M` into `(count, sides)` within sane bounds. A bare
+/// number `M` means `1dM`.
+fn parse_spec(s: &str) -> Option<(u64, u64)> {
+    let (count, sides) = match s.split_once('d') {
+        Some((c, m)) => {
+            let count = if c.is_empty() { 1 } else { c.parse().ok()? };
+            (count, m.parse().ok()?)
+        }
+        None => (1, s.parse().ok()?),
+    };
+    if (1..=MAX_DICE).contains(&count) && (2..=MAX_SIDES).contains(&sides) {
+        Some((count, sides))
+    } else {
+        None
+    }
+}