feat(sim): add dynamic coding rate policy

[Komzpa]

Summary

• add an opt-in Dynamic Coding Rate policy that selects LoRa CR 4/5..4/8 immediately before transmit
• use existing simulator context: local channel utilization, TX queue depth, retry state, ACKs, relay/last-hop context, direct relay context, and region duty-cycle limits
• keep ordinary first-attempt traffic compact; spend extra FEC on quiet retries, ACKs, non-busy direct relays, and last-hop relays
• add DCR TX packet and airtime histograms plus docs/tests

Stack

• split from feat(sim): add Batumi radio physics for DCR #64
• stacked after refactor(sim): make lora mesh cli importable #67, feat(sim): add map and terrain inputs #68, feat(sim): add land-cover clutter inputs #69, and feat(sim): add capture-aware rf physics #70; until the prerequisite PRs merge, GitHub will show their commits in this diff too
• intended review commit for this slice: b8939a3

Validation

• .venv/bin/python -m unittest tests.test_dcr tests.test_lora_mesh_cli tests.test_discrete_event_sim tests.test_node
• /home/kom/.local/bin/ruff check loraMesh.py lib/config.py lib/dcr.py lib/node.py lib/discrete_event_sim.py tests/test_dcr.py tests/test_lora_mesh_cli.py tests/test_discrete_event_sim.py tests/test_node.py
• .venv/bin/python -m compileall -q .
• .venv/bin/python -u loraMesh.py 2 --no-gui --simtime-seconds 5 --period-seconds 0.5 --dcr --phy-loss-model --capture-collision-model
• git diff --check
• .venv/bin/python -m unittest
• rg -n "stavnet|guest:guest|grafana|source_node_id|source_long_name|DTP|Dynamic TX Power" .

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