refactor(core): make local constants crate-private

Decision K (spec §5): the local capacities and timings are defaults that
will become configuration, so they are `pub(crate)`. The wire constants
stay public.

- MAX_PEERS, SUMMARY_INTERVAL_MS, REASSEMBLY_TIMEOUT_MS, SEND_QUEUE_CAP,
  MIN_MTU, EVENT_QUEUE_CAP, DROP_TABLE_CAP, BCAST_SENT_CAP, ACK_BUFFER_CAP
  and PEER_IDLE_MS are now `pub(crate)`; a compile_fail doctest pins it.
- MIN_MTU's doc claimed it was the floor below which a fragment has no
  room. It is only the frag sweep's lower bound (BLE ATT's 23). The real
  floors are now stated on `Transport::mtu` and pinned by
  `consts::tests::mtu_floors`: 85 to carry every valid observation, 66 to
  fragment at all, 35 for an Ack, 26 for a summary.
- tines-bt caps live connections with its own MAX_PEERS (8).
- tines-sim's props mirror MAX_PEERS/SEND_QUEUE_CAP, checked against core
  through the hidden `conformance` module; the fuzz twin reads it there.
- node.rs: drain_events' rustdoc no longer links the now-private consts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tines-core/tests/fuzz_twin.rs

@@ -22,7 +22,8 @@
 mod fuzz;
 
 use proptest::prelude::*;
-use tines_core::consts::{MAX_FRAME, MAX_PAYLOAD, MAX_PEERS, MAX_TOPIC, SUMMARY_BYTES};
+use tines_core::conformance::MAX_PEERS;
+use tines_core::consts::{MAX_FRAME, MAX_PAYLOAD, MAX_TOPIC, SUMMARY_BYTES};
 use tines_core::Store;
 
 // A single bounded case count keeps the twin fast under `cargo test` while