Split monolithic script into focused modules

Refactored the single 1200-line es24n_conf.py into six modules plus a
slim entry point, in preparation for the upcoming network-based config
workflow. Each file has a clear, single responsibility:

  ui.py              — ANSI colours, display primitives, input prompts
  serial_port.py     — SerialPort class (termios/fcntl/select)
  models.py          — IOMConfig and ShelfConfig dataclasses
  redfish.py         — Redfish API client (shared by all workflows)
  workflow_serial.py — Serial-based IOM network configuration workflow
  workflow_firmware.py — IOM and Fabric Card firmware update workflow
  es24n_conf.py      — Entry point and main menu only

No functional changes. All imports verified.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

models.py

@@ -0,0 +1,23 @@
+"""
+models.py — Shared data classes for ES24N IOM configuration.
+Used by both the serial and network configuration workflows.
+"""
+
+from dataclasses import dataclass, field
+
+
+@dataclass
+class IOMConfig:
+    iom:     str
+    dhcp:    bool = True
+    ip:      str  = ""
+    gateway: str  = ""
+    netmask: str  = ""
+
+
+@dataclass
+class ShelfConfig:
+    device:   str = ""
+    password: str = ""
+    iom1: IOMConfig = field(default_factory=lambda: IOMConfig("IOM1"))
+    iom2: IOMConfig = field(default_factory=lambda: IOMConfig("IOM2"))