Prepare Echo v0.12.4

[vriesd]

Outcome

Prepares the next release version for the selected-microphone capture fix and exact Small plus Large Turbo Linux acceleration packaging.

Changes

• Bumps all workspace packages and Cargo.lock from 0.12.3 to 0.12.4.
• Promotes the current Unreleased notes to v0.12.4.
• Adds the user-visible microphone shutdown fix to the release notes.
• Documents that non-admitted hardware stays on managed CPU.

Verification

• cargo check --workspace --locked
• python3 scripts/changelog-notes.py --self-test
• python3 scripts/changelog-notes.py v0.12.4
• git diff --check

Release gate

After merge, rebuild and requalify the exact v0.12.4 main executable for Small and Large on both Debian and RPM variants. Do not tag until the newly bound dual-admission packages pass deep verification.

[vriesd]

PASS

Independent release-metadata verification at 57ab4eb42b69eaee1493cbea6744d7e75b6caf5f.

• The workspace version is 0.12.4. echo, echo-core, echo-desktop, and xtask all resolve to 0.12.4 through workspace inheritance, and all four lockfile entries match.
• Normalizing those four version values back to 0.12.3 makes both Cargo.toml and Cargo.lock identical to main. No dependency changed.
• The only changed files are Cargo.toml, Cargo.lock, and CHANGELOG.md; there are no code, build-script, workflow, or runtime configuration changes.
• v0.12.4 notes cover both commits since v0.12.3: selected PipeWire shutdown capture and the exact dual-model admission/package path, including managed-CPU fallback and strict package rejection.
• The changelog parser self-test and v0.12.4 extraction passed. The head descends directly from current main, the tag name matches the workspace version, patch hygiene passed, and release-policy is green.
• Current PR SHA matched the tested worktree SHA. No merge action was taken.