heph.nvim: plug-and-play managed daemon (autostart, self-heal, client/server guardrail)

The plugin now manages its own hephd by default (autostart = true): if nothing
is serving the socket it spawns a local daemon against the default XDG paths,
kills only what it spawned on VimLeavePre, and self-heals — rpc.call retries
once through a respawn hook when the connection drops (the prior owner releases
the DB lock on exit, so a respawn can claim it).

- daemon.ensure() connects to an already-running daemon (any mode) or spawns one
  we own; stop_spawned()/is_managed() track lifecycle.
- A server/client daemon you started is always respected (spawn only when nothing
  serves the socket). autostart = false → connect-only, warns/errors if down,
  and clears the self-heal hook so it fails loudly.
- config: autostart defaults true; new `db` option; $HEPH_SOCKET / $HEPH_DB
  fallbacks isolate a dev Neovim onto a separate daemon + DB.

e2e: managed_daemon_spec covers autostart spawn, self-heal-after-kill, and
connect-only error. 10 specs green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

heph.nvim/lua/heph/rpc.lua

@@ -182,9 +182,34 @@ function M.session()
   return M._default
 end
 
---- Blocking call on the default session.
+--- Register a hook that (re)ensures the daemon — called once to self-heal a
+--- dropped connection before a single retry. `nil` disables self-heal (used when
+--- autostart is off, so a connect-only setup fails loudly instead of respawning).
+function M.set_respawn(fn)
+  M._respawn = fn
+end
+
+local function is_connection_error(msg)
+  msg = tostring(msg)
+  return msg:find("connect", 1, true) ~= nil
+    or msg:find("connection", 1, true) ~= nil
+    or msg:find("timeout", 1, true) ~= nil
+end
+
+--- Blocking call on the default session. If the call fails because the
+--- connection is dead and a respawn hook is set, ensure the daemon and retry
+--- once (the prior owner releases the DB lock on exit, so a respawn can claim it).
 function M.call(method, params, opts)
-  return M.session():call(method, params, opts)
+  local ok, result = pcall(M.session().call, M.session(), method, params, opts)
+  if ok then
+    return result
+  end
+  if M._respawn and is_connection_error(result) then
+    pcall(M._respawn)
+    M.session():close() -- drop the dead connection so the retry reconnects
+    return M.session():call(method, params, opts)
+  end
+  error(result)
 end
 
 --- An isolated session for a socket — used by tests for independent assertions.