fix(protocol): reassemble blockwise OBSERVE notifications

A notification carries only the first block of a large representation
(RFC 7959 §2.6). _dispatch_coap handed that block straight to
on_notification, so consumers decoded a truncated CBOR buffer. Reported
twice on /mode/vs/0: #37 and mbillow/localthings#361.

The recovery is a re-read from block 0 on a fresh 4-byte one-shot token,
not a §2.6 continuation. §3.4 rules out reusing the observation's token,
and this server drops a transfer that opens at NUM>0 under a token it
has not seen, so a continuation is the one shape that cannot work here.
A truncated notification is now withheld and queued to a worker thread
that re-reads the resource and delivers the reassembled representation.
When the re-read fails the notification is dropped at debug level and
the poll tiers carry freshness, which is what they already did.

The re-read has to run off the reader thread: _dispatch_coap runs there
and the transfer waits on an event only that same thread can set. The
worker is serialized and paces between transfers, so a notification
storm stays under the firmware request ceiling.

Also in the Block2 loop, now extracted and shared by both paths:

- compare the response's Block2 NUM against the one requested, so a
  retransmitted block is no longer concatenated as if it were the next
- compare ETags across blocks (§2.4) and restart once when the
  representation changes mid-transfer
- recompute the next block number from the accumulated byte offset when
  the server negotiates the block size down
- re-check reader liveness while waiting on a block, so a mid-transfer
  reader death fails fast instead of burning the whole timeout
- guard the token counters and _pending with a lock, now that the
  session issues concurrent reads of its own

Closes #39

tests/test_coap_wire.py

@@ -2,7 +2,8 @@ import pytest
 
 from smartthings_local.errors import MalformedMessageError
 from smartthings_local.protocol.coap import (
-    build_coap, parse_coap, encode_options, block_value, fmt_code,
+    build_coap, parse_coap, encode_options, block_value, block_fields,
+    fmt_code,
     TYPE_CON, METHOD_GET, URI_PATH, ACCEPT, CF_CBOR, BLOCK2,
 )
 
@@ -43,6 +44,23 @@ def test_block_value_promotes_to_two_bytes_when_num_is_large():
     assert len(v) == 2
 
 
+@pytest.mark.parametrize('num, more, szx', [
+    (0, 0, 0),
+    (0, 1, 6),
+    (2, 1, 6),
+    (1, 0, 4),
+    (0xFFF, 0, 0),
+    (0xFFFFF, 1, 7),
+])
+def test_block_fields_inverts_block_value(num, more, szx):
+    assert block_fields(block_value(num, more, szx)) == (num, more, szx)
+
+
+def test_block_fields_treats_empty_value_as_block_zero():
+    # RFC 7959 §2.2: a zero-length Block option means num=0, m=0, szx=0.
+    assert block_fields(b'') == (0, 0, 0)
+
+
 def test_fmt_code_formats_class_dot_detail():
     assert fmt_code(0x45) == '2.05'
     assert fmt_code(0x84) == '4.04'