refactor: extract shared utilities from duplicated MeshCore interface code

[devin-ai-integration]

Summary

MeshCore_Channel_Interface and MeshCore_Dynamic_Interface contained substantial duplicated logic. This PR extracts 10 shared utilities into a new Interface/_meshcore_shared.py module and rewires both interfaces to use them.

Extracted utilities:

Function / Class	What it replaces
load_meshcore(interface_name)	Identical _load_meshcore_or_panic import+log pattern in both files
start_asyncio_loop_thread(name, iname)	Identical _run_loop + threading.Thread boilerplate in both __init__
create_meshcore_connection(mc, transport, ...)	Duplicated serial/ble/tcp if/elif/else creation blocks (~30 lines each)
configure_radio(mc, freq, bw, sf, cr, ...)	Near-identical radio-parameter-override blocks
configure_channel(mc, idx, name, secret, ...)	Near-identical channel setup blocks
process_incoming(interface, data)	Identical 4-line processIncoming body (rxb += len; owner.inbound)
decode_tunnel_frame(text, prefix, ...)	Duplicated RNS: prefix-strip + base64url decode in _process_tunnel_text
reassemble_fragment(asm, meta, lock, key, ...)	Core fragment-reassembly logic (~25 lines each, same dict/lock pattern)
cleanup_stale_fragments(asm, meta, lock, ...)	Identical stale-assembly eviction in _cleanup_loop
PacketIdCounter(bits)	Replaces _pkt_id + _pkt_id_lock + manual & 0xFF / & 0xFFFFFFFF in both files

What stays in each interface:

• _PacketHandler classes (different wire formats: 9-byte vs 6-byte headers)
• Deduplication strategies (OrderedDict LRU in Channel vs sliding-window timestamps in Dynamic)
• Peer discovery / RNSBIND protocol (Dynamic only)
• RemoteTerm WebSocket support (Channel only)
• Rate limiting and RNS header parsing (Dynamic only)

Import mechanism for Reticulum's exec()-based loading:

try:
    _iface_dir = os.path.dirname(os.path.abspath(__file__))
except NameError:
    _iface_dir = os.path.expanduser("~/.reticulum/interfaces")
if _iface_dir not in sys.path:
    sys.path.insert(0, _iface_dir)

MeshCore_Interface.py is untouched — its older architecture (custom _safe_log, different fragmentation format, PacketHandler with Bb struct) is different enough that forcing shared code would reduce clarity.

Net diff: Channel −141 lines, Dynamic −37 lines, shared module +286 lines. The value is in eliminating duplicated patterns that would otherwise drift independently.

Link to Devin session: https://app.devin.ai/sessions/788f2435d2354a17b1474fcb1a261495
Requested by: @comms-engineer

[devin-ai-integration]

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