CWX: firmware never emits `cwx queue=` on drain — retire reply-radio_index workaround if it lands (fw 4.2.20 observed)

Summary

FlexRadio firmware does not emit a cwx queue= status when the CWX keyer buffer drains, so a client has no direct "queue empty" signal to release TX after a CWX macro/send finishes. With sync_cwx=1 the radio holds TX for its full hardware interlock timeout (~60 s) unless the client issues an explicit xmit 0 — but without queue= the client can't tell when to send it.

This is the root cause behind #3949 / #3979. The workaround there detects drain indirectly by capturing the radio_index from the final cwx send reply and watching the live cwx sent= counter reach the batch end (radio_index + nChars - 1).

Observed

• FLEX-6500, firmware 4.2.20.41343: cwx queue= is never sent during or after a CWX transmission (verified via the automation-bridge get cwx snapshot over multiple keyed macros — sent= advances per character, queue= never arrives).
• The reply radio_index is the batch's first-char (insertion-start) queue position, not the last char — the drain-watch end index must be computed as radio_index + nChars - 1.

Not yet confirmed on the 8600 / fw 4.2.18 (the nominal target), though both share the CWX protocol lineage.

Why this is upstream

If a future firmware starts emitting cwx queue= on drain (the natural, direct signal), the reply-radio_index watch in CwxModel/RadioModel becomes redundant and can be retired in favour of the simpler queue= path (a legacy fallback for it already exists in CwxModel::applyStatus). This issue tracks that retirement condition per the Optimistic Updates Policy.

Action if/when fixed upstream

1. Confirm cwx queue= (empty/0) fires on drain on the target firmware.
2. Re-enable the queue= → queueEmpty() path as primary; remove the reply-radio_index + nChars machinery in CwxModel/RadioModel and the get cwx cwxEndIndex/epoch fields.
3. Pin the firmware version at which the echo appeared.

Refs: #3949, #3979.

🤖 Generated with Claude Code

Start by verifying whether the target firmware emits an empty cwx queue= event on drain. Then inspect CwxModel::applyStatus, the reply-radio_index and nChars handling in CwxModel/RadioModel, and the get cwx cwxEndIndex/epoch fields; done means the queue path is primary and the workaround is removed once upstream behavior is confirmed.