feat: implement the B-RTR (Router) profile example

[sws-chipkin]

Summary

Implements the B-RTR (Router) profile - the series' canonical example for F-ROUTER and F-MULTIPORT. Seeded from BACnetProfileExample-B-ASC-CPP minus DeviceCommunicationControl, per the profile card.

• BIBBs: DS-RP-B, DS-WP-B, DM-DDB-A/B, DM-DOB-B, NM-RC-B (partial - see gaps below).
• Two Network Port objects ("Vermilion" / network 1, "Vermilion 2" / network 2), using common/ 2.3.0's multi-port SetupUDP/SendIAm overloads.
• Router configuration: AddRouterPort x2, AddRouterRoute (one demonstration route), SetRouterEnabled, SendIAmRouterToNetwork (bound to the new r key, common/ 2.4.0), SendWhoIsRouterToNetwork, SendNetworkNumberIs.
• Pinned to CAS BACnet Stack 6.x @ abd4cee1 (6.0.21), STATIC link. Vendors common/ 2.5.0.

Known gaps (documented in TODO.md, filed upstream)

1. DM-LM-B (AddListElement/RemoveListElement, services 8/9) not implemented. The customer-facing STATIC build does not compile in STACK_OPTION_DM_LM_LIST_MANIPULATION. chipkin/cas-bacnet-stack#2033
2. Inter-network NPDU forwarding does not work. The pinned stack holds exactly one internal datalink instance per network type; two BACnet/IP router ports cannot both attribute inbound traffic, so nothing is actually forwarded between the two networks even though AddRouterPort/AddRouterRoute/SetRouterEnabled all succeed and Network_Number/Network_Number_Quality read back correctly. Root-caused to a scope gap in stack issue #304 (closed, but implemented for BACnet/SC only). chipkin/cas-bacnet-stack#2037
3. Routing_Table cannot be read. Enabling it and reading it aborts the request - the stack has no UseCallbackGetList case for it outside a test-tool build. Not enabled in this example. chipkin/cas-bacnet-stack#2038

None of these are faked or worked around; the README's "Verify" section and a prominent notice at the top state plainly what does and does not work.

Verification

• STATIC build (VS2022/v143 toolset), zero warnings/errors from main.cpp/common/.
• Live-client wire verification (bacpypes3), against a running instance:
  • ReadProperty of the Device object (Object_Name = "Rainbow") directly on BOTH ports independently.
  • Network Port 1: Network_Number = 1, Network_Number_Quality = configured, Object_Name = "Vermilion".
  • Network Port 2: Network_Number = 2, Network_Number_Quality = configured, Object_Name = "Vermilion 2".
  • WriteProperty Analog Output 1 Present_Value = 25.5 @ priority 8, readback confirmed.
  • I-Am-Router-To-Network broadcast confirmed on both ports via the device's own TX log, at start-up and on the r key.
• tools/check-series.sh BACnetProfileExample-B-RTR-CPP → SERIES CHECK PASSED.
• Self-history grep: clean (no hits outside vendored common/'s pre-existing, already-reviewed comments).
• docs/objects.json / generated "Objects and properties" README block: 0 ⚠ rows.

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