Introduce ModeStack, a pure-Rust refcount-composable wrapper for
Android audio-mode acquire/release with prior-mode snapshot. Per
SDD-108 §1/§2 the engine snapshots the system audio mode on first
acquire and restores it on last release; composed acquires are
no-ops while the mode is held.
ModeStack is panic-free; release-on-zero returns AlreadyReleased
rather than panicking. Six SWE4-UV-045-tagged unit tests cover the
acquire/release semantics on the host target.
Trace: SDD-108, SWE4-UV-045.
EdisonJwa
da0b208075