feat(client): T-1132 standalone Atlas companion shell + make atlas (D-254 SS3)

New atlas_standalone scene/script: attach-or-spawn boot (one REAL
connect_to_sim attempt at SR_PORT-or-9876 — a separate throwaway TCP
probe was proven by live run to kill a pre-accept-loop server via
broken handshake pipe; never abandon a connected socket), else spawn
--port 0 via new ServerProcess.start_with_pipe + LISTENING:{port}
stdout parse, retry against the resolved port. Reader role wired end
to end: protocol.encode_startup_message optional role param (empty
omits the wire key — byte-identical for all existing callers),
sim_bridge.connection_role suppresses the post-handshake
RequestAllSettings auto-send, hud_groups skips AutoPause/AutoResume
sends for readers (all three would otherwise burn Reader violation
strikes per the T-1130 matrix — endorsed by Oscar).

Generic implant host per D-254 SS3: the shell instantiates ALL
registered implant apps; implant_app_manifest gains
available_in_companion (opt-out, default true) and
implant_registry.instantiate_all a standalone filter param (default
preserves hud.gd behavior byte-identically). Boot order is
instantiate_all THEN open_app (reverse renders a permanently black
window — app_changed fires with no listener; matches hud.gd's order).
Owned-server lifecycle: _exit_tree stops a spawned child, attached
servers survive companion close. Known engine limitation documented:
raw SIGTERM bypasses all Godot notifications and orphans a spawned
server; WM close paths verified clean.

Live-verified: spawn-mode (301 systems rendered from systems.db over
the wire), attach-mode, two simultaneous readers, clean shutdown with
zero orphan processes. 14 new gdUnit tests (port/LISTENING parsing).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/scripts/protocol/server_process.gd

@@ -2,6 +2,9 @@ class_name ServerProcess
 ## Manages the Rust simulation server as a subprocess (D-020).
 
 var _pid: int = -1
+## Populated only by start_with_pipe() — the stdout FileAccess handle used to
+## read the LISTENING:{port} signal (D-254 §1/§3). Null after plain start().
+var _stdio: FileAccess = null
 
 
 ## Start the server process. Returns the PID, or -1 on failure.
@@ -19,6 +22,43 @@ func start(server_path: String, args: Array = []) -> int:
 	return _pid
 
 
+## Start the server process with its stdout piped back to this process
+## (D-254 §1/§3: the companion app's --port 0 spawn-mode, which needs to read
+## the LISTENING:{port} signal main.rs prints — see server/src/main.rs). Unlike
+## start()/OS.create_process, this uses OS.execute_with_pipe, which does not
+## block (the shell precedent, tests/run-visual, achieves the same read via a
+## redirected log file + `grep` from a wrapper script; this is the Godot-native
+## equivalent for a caller with no shell wrapper). Returns the PID, or -1 on
+## failure. Call read_stdout_line() from a per-frame poll to drain the pipe.
+func start_with_pipe(server_path: String, args: Array = []) -> int:
+	if _pid > 0 and is_alive():
+		push_warning("ServerProcess: server already running (pid %d)" % _pid)
+		return _pid
+	if not FileAccess.file_exists(server_path):
+		push_error("ServerProcess: server binary not found at %s" % server_path)
+		return -1
+	var result: Dictionary = OS.execute_with_pipe(server_path, args)
+	if not result.has("pid") or int(result.get("pid", -1)) <= 0:
+		push_error("ServerProcess: failed to start server (pipe) at %s" % server_path)
+		return -1
+	_pid = int(result["pid"])
+	_stdio = result.get("stdio", null)
+	return _pid
+
+
+## Non-blocking read of one line from the piped server's stdout, or "" if no
+## complete line is buffered yet (FileAccess.get_line() on a pipe never
+## blocks — it returns immediately with whatever is currently available).
+## Only meaningful after start_with_pipe(); returns "" if _stdio was never set
+## (plain start()) or the process has exited (stream at EOF).
+func read_stdout_line() -> String:
+	if _stdio == null:
+		return ""
+	if _stdio.get_error() != OK:
+		return ""
+	return _stdio.get_line()
+
+
 ## Check if the server process is still running.
 ## NOTE: OS.is_process_running() doesn't distinguish starting/healthy/zombie.
 ## TODO: Add health check (e.g. TCP readiness probe) when server supports it.