audio: record-only mode via negative audioPort

[snokvist]

Summary

Adds a way to enable + record audio without streaming it, by making outgoing.audioPort a signed tri-state:

audioPort	Behavior
< 0 (e.g. -1)	Capture + record only, never stream (new)
0	Share the video target (unchanged)
> 0	Dedicated UDP audio port (unchanged)

The capture/encode thread still runs and feeds the recording ring on the record-only path; only the network send is skipped. Recording is gated on record.enabled/audio.enabled, independent of audioPort, so audio still lands in TS-format recordings.

Motivation: previously there was no clean knob for this. audio.enabled=false kills recording too; audio.mute silences the source; audioPort=0 mixes audio onto the video stream (UDP) or fully disables audio in SHM mode. A negative value cleanly expresses "capture + record, never transmit" without depending on the video transport.

Changes

• Widen audio_port uint16_t → int32_t (venc_config.h, maruko_config.h)
• Drop the truncating (uint16_t) parse cast; render via pp_field_int
• FIELD type FT_UINT16 → FT_INT (accepts negatives via strtol, still MUT_RESTART); settable via config file or /api/v1/set?outgoing.audioPort=-1
• stream_disabled flag on both audio backends (star6e_audio.c, maruko_audio.c); skip output-socket init and guard both send sites; record-only (no stream) init log
• Test: -1 survives parse + save/reload round-trip
• Docs (README.md, AUDIO_UDP_OUTPUT_FEASIBILITY.md) + dashboard tooltip + regenerated venc_webui.c

Constraints

• Record-only audio only works with a TS-family record.format — HEVC recordings carry no audio track (pre-existing).

Verification

• make test-werror → 1659 passed, 0 failed (incl. new -1 parse + round-trip)
• make verify → passed end-to-end (Star6E + Maruko builds + webui-check)
• Device-verified on Star6E (SHM video, opus audio): audioPort=-1 produces no audio socket on the wire while the .ts recording contains the opus track (3527 packets / 72.7 s). Also verified the runtime API set: /api/v1/set?outgoing.audioPort=-1 applies, persists, and respawns into record-only mode.

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