Add --fixed-volume for an output whose level is set elsewhere

[chrisuthe]

Adopts sendspin-cpp#203 (set_volume_adjustable / set_mute_adjustable, in the v0.9.5 pin) as one switch, --fixed-volume / fixed-volume = true, for an output whose level is set elsewhere, such as a line-out into an amplifier.

What changes

• Volume and mute are withdrawn together. With the flag the role reports neither volume nor muted in client/state and lists neither command in supported_commands, so a server's slider for this player disappears rather than silently doing nothing; a command for either is ignored by the library. A mute the server could still set would silence a fixed level, which is why the two go together.
• The sink plays at full gain, unmuted. PlayerListener::fix_volume() pins the sink and marks the source VolumeSource::Fixed; update_volume(100) / update_muted(false) keep the role reporting what the sink applies. The listener's on_volume_changed / on_mute_changed are guarded while fixed, so a command can never reach the sink or the state file.
• A remembered volume is kept, not restored. One INFO line says the remembered value is ignored while the flag is set and returns once it is dropped. The state file is not rewritten.
• status prints player volume: 100 (fixed; the server cannot set it).
• vol and mute on the control socket are untouched. They are group commands to the server, refused only on what the server publishes.
• Docs: --help, README, the config key table, Controlling the Player, and the example config.

Verification

• ctest: 519/519 apart from ConfigMerge.AConfiguredControlSocketIsAbsolutizedUnderDaemonize, which fails only because this worktree's socket path is 105 bytes; it passes from a short directory.
• New tests: the flag and key (true, false, maybe refused with the allow-unpaired wording), the status note verbatim, and a listener test that with a remembered volume a server volume or mute command under the flag leaves the state file byte-identical, next to one showing the same commands apply and persist without it.
• scripts/format.sh --check, scripts/smoke_test.sh and a -DSENDSPIN_CLI_WERROR=ON build are clean.
• Run by hand with volume = 40, muted = true in the state file: with the flag, the log names the ignored value, status prints the fixed note and the file is byte-identical after the run; without it, Restored volume 40 (muted) and status read as before.