Rewrite Region/Scope design to match actual MeshCore protocol

The previous design was based on an allow-list model with geographic
ISO codes. The actual MeshCore protocol uses a deny-based RegionMap
with hierarchical names and REGION_DENY_FLOOD flags on transport
code packets.

New design:
- Deny-based filtering (default: forward everything)
- Flat RegionMap with 4 entries (simplified for HTCC-AB01 constraints)
- Transport code extraction from TRANSPORT_FLOOD/DIRECT packets
- MeshCore-compatible CLI command reference
- Flash budget analysis (~900 bytes available)

Author: atomozero