Mob can verify visual output in-process via screenshot/3, but had no
equivalent for audio. This adds two read-only probes that answer "is
sound actually coming out right now":

- Mob.Audio.output_status/0 — {volume, muted, route, other_audio}.
  Cheap, no permission. Catches the common silence causes (muted,
  volume 0, dead route). iOS AVAudioSession / Android AudioManager.
- Mob.Audio.output_level/1 — {rms_db, peak_db} | :silent | {:error, _}.
  Reads actual signal energy, the part output_status and `adb dumpsys
  audio` cannot answer. source: :mix (default) taps the global output
  mix so it sees native players that bypass Mob.Audio (e.g. a game's
  own AudioTrack); source: :mob taps Mob.Audio's own player.

Native wiring mirrors screenshot/3 + open_settings/1: -export/-nifs/stub
in mob_nif.erl, native table entries in mob_nif.zig and mob_nif.m, and
cacheOptional + null-guard for the app-owned Android bridge methods so a
drifted MobBridge.kt no-ops instead of crash-looping boot. iOS level-2
uses AVAudioPlayer metering (self-contained); Android uses a Visualizer
on session 0 (needs RECORD_AUDIO). output_level is a dirty IO NIF.

Honest asymmetry: for audio that bypasses Mob.Audio, level-2 works on
Android (global-mix tap) but not iOS (sandbox forbids it) — documented,
not papered over. Decoders are pure Elixir and unit-tested on host; the
Android Kotlin bridge methods ship in the mob_new template (separate PR).

Host: 969 mob tests pass, formatters clean. Device verification (boot +
live level reads) is the gate for the native paths.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

Device verification on a moto g power (2021), Android 11, corrected the
design: a session-0 (global output mix) Visualizer fails with ERROR_NO_INIT
for a normal app even with RECORD_AUDIO + MODIFY_AUDIO_SETTINGS — global
output capture is privileged. So the global-mix `:mix` path can't work in
the core framework.

Revised output_level/1:
- default source is now :mob — meters Mob.Audio's own player. iOS via
  AVAudioPlayer metering; Android via a Visualizer on the player's OWN
  audio session (audioPlayer.audioSessionId), which works with RECORD_AUDIO.
- :mix returns {:error, :unsupported_on_platform} on both platforms. True
  global/foreign-app capture is deferred to a separate MediaProjection-based
  plugin (test-env dep) — see the decision doc.

The Android NIF now decodes a length-coded return (float[2] = level;
float[1] = error code 1 unsupported / 2 needs_record_audio / 3 not_playing).

Verified on hardware (dist-RPC into doom_demo):
  output_status      → %{route: :speaker, volume: 0.2, muted: false}
  output_level(:mob) → {-34.8, -31.8} playing; {:error, :not_playing} idle
  output_level(:mix) → {:error, :unsupported_on_platform}
  no RECORD_AUDIO    → {:error, :needs_record_audio}
  app boots cleanly (NIF table OK)

output_status/0 is unchanged and fully verified. Companion template update:
mob_new (own-session Visualizer, no session-0 tap).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

Integrates mob#67 (input-level metering, now on master) with the output
probes on this branch:
- audio.ex: drop the duplicate decode_level/1 the merge produced (the
  shared helper now lives once; output_level + input_level both use it).
- audio_test.exs: keep decode_status/1 tests; unify decode_level/1 to cover
  atoms from both probes (:not_playing/:needs_record_audio, :not_metering,
  :unsupported_on_platform).
- ios/mob_nif.m: forward-declare g_audio_player so output_level/1 compiles
  (it referenced the play/1 player ~620 lines before its definition — a
  latent iOS build break independent of the merge).

Elixir compiles --warnings-as-errors; 30 audio tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>