OpenCyphal / libcanard

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Fix RX behavioral defects: reassembly asserts, filter config, callback re-entrancy (#260)

## Summary

v5 TX/RX hardening in two parts. Every fix ships with a regression test
that fails on the pre-fix code and passes after.

### RX behavioral fixes (`canard.c`)

- `rx_subscribe`: accumulate `filters_dirty` (`||`) — a duplicate
subscribe no longer cancels a pending acceptance-filter reconfiguration
(was silent RX loss on filter-capable nodes).
- `rx_session_accept` / `rx_session_complete_slot`: drop the false
`slot->iface_index == ses->iface_index` assert that aborted on valid
redundant-interface traffic (per-slot affinity is still enforced at
admission).
- `rx_parse` (v0 anonymous): route on the 2 low data-type-ID bits per
the DroneCAN spec (the upper bits are a random discriminator), with a
weak HW acceptance filter for v0 DTID ≤ 3 — still one filter entry per
subscription.
- `rx_session_update`: dispatch `on_message` last, so a callback that
unsubscribes itself can't cause a use-after-free.
- `canard_new`: set `filters_dirty = filter_count > 0` so forced
Heartbeat/NodeStatus occupancy filters program even before the first
subscription.
- Forced-filter dedup: replace the alias-prone single-frame probe with a
filter-coverage (subset) test `rx_filter_covered`.

### Available-interfaces bitmap (intentional `canard_new` API break)

`canard_new()` gains an `iface_bitmap` of the interfaces available on
the node, ANDed into every enqueue at the single TX chokepoint
(`tx_push`). Requests/responses and all-publishes no longer queue frames
for interfaces that can't be drained. A **zero** bitmap declares a
**listen-only** node (TX is a clean no-op). Bits beyond
`CANARD_IFACE_COUNT` are rejected. Call sites, the demo, and the README
example are updated.

## Verification

Full suite green in the Debug build (assertions on); `clang-tidy` +
`cppcheck` + `clang-format` clean. Each fix was confirmed to be a real
guard by reverting its hunk and watching the test fail ... (continued)

218 of 218 branches covered (100.0%)

Branch coverage included in aggregate %.

31 of 31 new or added lines in 1 file covered. (100.0%)

880 of 880 relevant lines covered (100.0%)

14657.51 hits per line