Route all mesh message sizing (renderer, digest, reply chunker, NWS
one-packet fit) through the active transport's max_chars instead of
scattered literal 200s. Meshtastic pinned at 200 (byte-identical output);
MeshCore uses its configured ~140. Sets up uniform-to-smaller sizing for
the composite transport. No behavior change on the Meshtastic path.
Co-authored-by: Matt Johnson <mj@k7zvx.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
