ffi/readline: blocking readline() + add_history(), nil on EOF

Phase 0 binding per PHASE0.md §9. M.readline(prompt) returns the line
as a Lua string (the C buffer is freed via libc free immediately after
ffi.string copies it) or nil on EOF. M.add_history skips empty lines.

Loader handles the case where libreadline-dev's unversioned
`libreadline.so` symlink isn't installed — falls through to
`readline.so.8` (current Debian/Arch ALARM) and `.so.7` (older)
before giving up. This trips on noether-the-LXD: only the runtime
package is present.

Smoke (stdin from heredoc, two lines + EOF):

  p1> hello world  -> "hello world"
  p2> second line  -> "second line"
  p3>              -> nil (EOF)

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

ffi/readline.lua

@@ -1,5 +1,5 @@
 -- ffi/readline.lua — GNU readline binding.
--- Phase 0: readline + add_history + free. Phase 1: custom key bindings.
+-- Phase 0: readline + add_history + EOF handling. Phase 1: custom key bindings.
 -- See docs/PHASE0.md §9.
 
 local ffi = require("ffi")
@@ -10,6 +10,41 @@ void  add_history(const char *line);
 void  free(void *ptr);
 ]]
 
+-- libreadline-dev (which ships the unversioned `libreadline.so` symlink) is
+-- not assumed to be installed on the runtime host; fall back to versioned
+-- sonames so a base Debian/Arch with just libreadline runtime works.
+local function load_readline()
+    local errs = {}
+    for _, name in ipairs({"readline", "readline.so.8", "readline.so.7"}) do
+        local ok, lib = pcall(ffi.load, name)
+        if ok then return lib end
+        errs[#errs+1] = name .. ": " .. tostring(lib)
+    end
+    error("libreadline not loadable: " .. table.concat(errs, "; "))
+end
+
+local rl = load_readline()
+local C  = ffi.C
+
 local M = {}
--- Phase 0 stubs; wired with the REPL implementation.
+
+-- Read one line of input.
+-- Returns:
+--   string : the line (no trailing newline)
+--   nil    : EOF (Ctrl-D on empty line)
+function M.readline(prompt)
+    local cstr = rl.readline(prompt)
+    if cstr == nil then return nil end
+    local s = ffi.string(cstr)
+    C.free(cstr)
+    return s
+end
+
+-- Append a non-empty line to readline's in-memory history.
+function M.add_history(line)
+    if line and #line > 0 then
+        rl.add_history(line)
+    end
+end
+
 return M