Fix DDP fragmentation: add multi-packet handling and data offset support

[andrewjswan]

Description

This PR resolves an issue where addressable LED strips with more than 480 pixels stop rendering after the first 480 LEDs. It addresses the bug where incoming fragmented DDP packets with a non-zero data offset were either ignored or caused memory mapping corruption.

Changes

• ddp.cpp: Extracted 32-bit data_offset from DDP header bytes 4-7, converted byte offset to LED index, and passed it down to the effect processing chain.
• ddp_light_effect_base.h: Modified interface definition of process_() method to support an optional offset parameter defaulting to 0.
• ddp_addressable_light_effect.cpp:
  • Updated packet length validation to account for the current offset position.
  • Replaced the high-overhead (i - used) / 3 division inside the performance-critical loop with a fast pointer-style target_led++ increment.
  • Implemented boundary check via break statement to prevent hardware buffer out-of-bounds access.
• ddp_light_effect.cpp: Added an early exit rule for packets where offset > 0 to ensure single-bulb components ignore stream fragments meant for addressable strip installations.

Verification

• Tested with Hyperion.ng driving a strip of 900 addressable LEDs.
• Verified that logs no longer output "Ignoring DDP Packet with non-zero data offset".
• Confirmed full-length strip coverage (all 900 pixels respond to DDP stream correctly without flicker or ghosting).