interval: make refresh override reversible

[barlevalon]

Behavior

• Temporarily lowers status-interval only while scrolling is enabled, automatic interval control is enabled, and rendered metadata exceeds the scrolling threshold.
• Restores the captured interval and clears ownership state for short or empty output and when either controlling option is disabled.
• Captures a fresh user-selected interval on each later acquisition instead of restoring stale state.
• When status-interval changes externally during an active override, records that value as the new restoration target before reapplying the temporary interval.
• Serializes concurrent status refresh transitions and ignores unpaired legacy ownership markers, preventing inconsistent or stale restoration.

Structural simplification

• Consolidates interval acquisition and release in update_nowplaying_status_interval.
• Removes the separate original-interval getter and empty-output restoration wrapper.
• Initializes empty track state and invokes lifecycle handling exactly once per render.
• Uses paired original/applied markers to distinguish current ownership from stale state.

Tests

• Replaces the stateless tmux stub with a persistent file-backed mock supporting global reads, writes, unsets, and wait-for locking.
• Covers acquisition, repeated acquisition, short and empty output, disabled controls, cleanup, inactive no-ops, later user interval recapture, changes made during active ownership, stale legacy state, and deterministic active/inactive interleaving.
• Passes shell syntax checks, the full shell test suite, ShellCheck, and isolated real-tmux lifecycle checks.

Residual risks

• A process killed without running its EXIT trap after acquiring the wait-for lock could leave that lock held.
• External tmux commands do not participate in the plugin lock, though observed interval changes are preserved on the next lifecycle transition.

Integration

• consolidated the shared stateful tmux mock while preserving absent-vs-empty option fallback and status interpolation coverage
• added isolated real-tmux coverage for 15 -> 1 -> external 5 -> active refresh -> release 5, including marker cleanup