N1: linkDown is set only when NO reply arrived, i.e. a timeout, a transport exception or
a connect failure. A status the CNC returned proves it answered. EW_REJECT and every
unlisted rc map to BadCommunicationError, so a deterministic rc on one tag used to fail
every later tag of a reachable control on every poll. The comms-classed status still
degrades health. The coalescer already treated a returned refusal this way; it is
unchanged.
Tests: a returned EW_REJECT on tag 1 leaves tags 2 and 3 read and Good (red before). A
timeout still fails the rest without reading them (positive control).
Minors:
- m1: the _lifecycleLock doc now covers the health transitions it also guards.
- m2: the NoteFixedTreeFailed summary says a refusal credits the answer.
- m3: ReadAsync stamps a failed batch's exception with the generation the batch started
under, and HandlePollError degrades that generation only.
- m4: the fixed-tree "blip" absence check runs unconditionally under a wide budget, and
asserts that both ticks landed inside it.
- m5: direct unit cases for the coalescer link-down paths. A transport fault on the first
range fails the second range and the per-tag tag without reading them. A connect
failure costs one connect per batch. CncRelay.Up() retries the re-bind.
Refs #740
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty