LKMA-196: Default SIPfax to spandsp worker

[nicolasumaras]

Summary

• Default production modem startup to the spandsp soft-modem worker via SIPFAX_SOFTMODEM_BINARY, with SIPFAX_MODEM_COMMAND remaining an override.
• Preserve worker control-fd snapshots in media.modem diagnostics and log backend errors.
• Update deployment docs/env example and add regression coverage that a missing worker emits no synthetic RTP.

Verification

• npm test

Not run

• Live SIPfax VM call verification; needs deployed build on the VM to confirm real media.modem modulation diagnostics.

[nicolasumaras]

Reviewer: Approved (LKMA-196)

GitHub prevents formal approval of your own PR — recording verdict here instead.

Verdict: Approved

PR Checklist

Item	Result
Head branch	lkma-196-default-spandsp-worker — task-specific single commit
Head commit reviewed	3fa3569b987b85334d36aac1f1dd8783e276b878
Required CI check (test)	SUCCESS
PR mergeability	CLEAN / MERGEABLE

Deliverables Verified

src/index.js — createModemBackend() unconditionally returns ExternalModemProcessBackend. Command resolves as SIPFAX_MODEM_COMMAND ?? softmodemBinary; default path /opt/sipfax/bin/sipfax-softmodem. InProcessDialupTerminator import removed. ✅

src/media.js — lastControl object initialized (state: 'idle') and spread-merged on each fd 3 JSON line with lastEventAt timestamp. diagnostics() exposes modulation, baud, state, ber, lastEvent, lastEventAt, lastError. writeInboundAudio guards on stdin.destroyed || !stdin.writable. ✅

src/server.js — backend-error events logged to stderr. ✅

Deploy/README docs — SIPFAX_SOFTMODEM_BINARY documented; SIPFAX_MODEM_COMMAND demoted to override role; both README files updated. ✅

test/session.test.js — New "unavailable worker emits no synthetic RTP" test: after ENOENT backend-error, inbound RTP produces zero bridge frames out, modem.type === 'external-process', lastError matches /ENOENT/. All 27 tests pass. ✅

InProcessDialupTerminator / ModemAnswerToneSource — Remain in src/media.js for test fixture use; no longer imported/wired in src/index.js. ✅

Non-blocking follow-up

Live VM call verification (confirming media.modem.modulation shows real worker state, not synthetic answer-tone) logged as a separate deployment validation task.