feat: adaptive snap release for upward compressor stage

[Copilot]

During silence, the upward stage's gainEnv accumulates a large boost. When a transient arrives, targetGainDb drops to zero immediately but gainEnv decays at the normal (slow) release rate — amplifying the transient for several milliseconds. Fix: detect the silence-to-transient event (>6 dB downward jump in target) and substitute a fast "snap" release coefficient for that decay cycle.

DSP (CompressorStage.h)

• Added snapReleaseMs / snapReleaseCoeff / snapReleaseEnabled state
• updateCoefficients() now precomputes snapReleaseCoeff
• Ballistics step selects snap coeff when upward stage is releasing and genv − targetGainDb > kSnapReleaseThresholdDb (6 dB, hardcoded detection threshold — not user-visible)

if (!attacking) {
    if (direction == StageDirection::Upward
        && snapReleaseEnabled
        && (genv - targetGainDb) > kSnapReleaseThresholdDb)
        coeff = snapReleaseCoeff;
    else
        coeff = releaseCoeff;
} else {
    coeff = attackCoeff;
}

Parameters (PluginProcessor.h/.cpp)

• up_snap_release_enabled — AudioParameterBool, default off
• up_snap_release — AudioParameterFloat, 0.5–50 ms, default 5 ms
• Both pushed to CompressorStage each block via OttCompressor delegating setters

UI (PluginEditor.h/.cpp)

• "Snap Rel" toggle + "Snap Rel (ms)" slider added to the Upward group
• Slider/label hidden when toggle is off; visibility correctly initialised from loaded parameter state on plugin open
• Upward group resizes dynamically: 6 rows (toggle off) or 7 rows (toggle on)

[huberp]

@copilot revert this PR from main and apply it to branch "release-0.5.0"