readline: bind() via rl_bind_keyseq; repl reserves \C-n no-op

Phase 1 readline binding wiring per PHASE1.md §7.

ffi/readline:
  M.bind(seq, lua_fn) -> bool
    Wraps lua_fn as a C callback (signature `int (int, int)` per
    readline's rl_command_func_t) and registers it via
    rl_bind_keyseq(seq, cb). Returns true on success (rl returns 0).
    Trampolines are pinned in module-local state so they outlive the
    bind call — readline retains the function pointer for the process
    lifetime. Rebinding the same seq frees the previous trampoline.
    Bound handlers are pcall-wrapped so a Lua error doesn't crash
    readline's input loop.

repl:
  Binds \C-n to a no-op that emits
    "[aish] Norris mode not yet implemented (Phase 3)"
  Verifies the mechanism end-to-end; Phase 3 (Norris autonomous mode)
  replaces the body with the actual toggle.

Smoke covers bind / rebind-same-seq (exercises the :free path) /
bind-different-seq with no errors. Live keyboard verification waits
on user-test.

Phase 1's 8(+1) inner loop is now functionally through `implement`;
next inner phase is `verify` (review pass) followed by memory-update.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ffi/readline.lua

@@ -1,6 +1,7 @@
 -- ffi/readline.lua — GNU readline binding.
--- Phase 0: readline + add_history + EOF handling. Phase 1: custom key bindings.
--- See docs/PHASE0.md §9.
+-- Phase 0: readline + add_history + EOF handling.
+-- Phase 1: custom key bindings via rl_bind_keyseq.
+-- See docs/PHASE0.md §9 and docs/PHASE1.md §7.
 
 local ffi = require("ffi")
 
@@ -8,6 +9,9 @@ ffi.cdef[[
 char *readline(const char *prompt);
 void  add_history(const char *line);
 void  free(void *ptr);
+
+typedef int (*rl_command_func_t)(int, int);
+int rl_bind_keyseq(const char *keyseq, rl_command_func_t function);
 ]]
 
 -- libreadline-dev (which ships the unversioned `libreadline.so` symlink) is
@@ -47,4 +51,26 @@ function M.add_history(line)
     end
 end
 
+-- Bind `seq` (e.g. "\\C-n") to a Lua function that runs when the user types
+-- that key sequence at the readline prompt. The Lua fn takes no arguments
+-- (readline passes count + key, but Phase 1 consumers don't need them).
+-- Callback trampolines are pinned in module-local state so they outlive the
+-- M.bind call — readline retains the function pointer indefinitely.
+local _bound = {}
+
+function M.bind(seq, fn)
+    if _bound[seq] then
+        _bound[seq]:free()
+    end
+    local cb = ffi.cast("rl_command_func_t", function(_count, _key)
+        local ok, err = pcall(fn)
+        if not ok then
+            io.stderr:write("ffi/readline bind handler error: " .. tostring(err) .. "\n")
+        end
+        return 0
+    end)
+    _bound[seq] = cb
+    return rl.rl_bind_keyseq(seq, cb) == 0
+end
+
 return M