Fix installer Linux audio dependencies

[andy5090]

What changed

• Detect missing ALSA runtime packages on Debian and Ubuntu during installation.
• Install libasound2 and libasound2-plugins through apt-get when needed.
• Document the WSLg PulseAudio requirement and complete source-build dependencies.

Why

Termleaf uses Rodio/CPAL's ALSA backend on Linux. WSLg exposes its Windows audio
route through PulseAudio, but a minimal Ubuntu installation may not include the
ALSA PulseAudio plugin. In that case Termleaf cannot open a default output stream
and runs silently.

The installer now supplies the missing runtime dependency instead of leaving the
application installed without working audio.

Validation

• sh -n termleaf-installer.sh
• git diff --check
• cargo test --locked (84 passed, 1 hardware test ignored)
• cargo test --locked audio::tests::hardware_audio_callback_stays_alive -- --ignored --nocapture
• WSLg playback of assets/typewriter-return.wav with aplay

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Install libasound2t64 on Ubuntu 24.04

On a minimal Ubuntu 24.04 (Noble) installation—the WSLg scenario this change targets—libasound2 is a virtual package rather than an installable package. I checked apt-get install --simulate --yes libasound2 on Noble, which reports that it is provided by libasound2t64 and then fails with Package 'libasound2' has no installation candidate. Consequently, when ALSA is absent, the new dependency step exits under set -e before Termleaf is downloaded; detect/install libasound2t64 on systems where that is the concrete package, or otherwise resolve the available provider.

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