implement adaptive bit reservoir and PE-driven rate control

[nschimme]

This PR introduces an adaptive bit reservoir and dynamic rate control system to deliver significantly improved ABR targeting and overall bitrate accuracy.

Why?

The existing rate control relies solely on a static dampener, leading to three major issues in dynamic or non-standard encoding scenarios:

1. Low-Bitrate Reservoir Exhaustion: Without adaptive capacity limits or draw caps, complex frames deplete the bit reservoir too early, forcing sudden, aggressive quality drops in subsequent frames to stay under target.
2. Inadequate Transient Funding: Without a mechanism to detect transient bursts, the open-loop dampener fails to allocate adequate bits to high-entropy frames, causing localized distortion on sharp attacks.
3. Slow ABR Convergence: Master struggles to hit target bitrates quickly on short audio clips or non-44.1 kHz sample rates, drifting from the requested target.

This PR introduces an ISO-compliant adaptive bit reservoir paired with closed-loop feedback control to address these issues:

• Higher Accuracy: Replaces open-loop dampening with proportional error correction (Kp = 0.08) and dynamic damping to eliminate target drift.
• Transient & Bit-Drain Protection: Gates reservoir draws using fast perceptual entropy estimation so transient attacks receive extra bits without depleting the pool during steady states.
• Instant Convergence: Uses sample-rate-aware seeding so ABR stabilizes in just 1–2 frames, regardless of input sampling rate.
• Zero Quality Penalty: Achieves these stability gains and a throughput boost without regressing audio quality.

Key Changes & Algorithm Choices

• Fast Subblock Energy PE (blockswitch.c): Approximates Perceptual Entropy (PE) using pre-computed subblock high-pass energies scaled by 1.0e-8f while gating reservoir draws (PE > 10.0 * channels) to properly fund transient attacks.
• ISO Reservoir Sizing (frame.c): Caps capacity at min(2 * desbits, 6144 * channels - desbits) and restricts single-frame draws (desbits / 2 for <=24 kbps; desbits for higher) to prevent low-bitrate reservoir exhaustion.
• Closed-Loop Feedback (frame.c): Stabilizes quality using proportional reservoir error correction (Kp = 0.08), fill-dependent dynamic damping (0.60 -> 0.85), a 0.5% update deadband, and [0.80, 1.20] quality scale factor clamping.
• Sample-Rate Seeding (frame.c): Scales initial quality seeds by 44100 / sampleRate to achieve rapid 1–2 frame ABR convergence on short audio clips across all sample rates.

Empirical CI Benchmarks

Metric	Value / Impact	Notes
Bitrate Accuracy	88.2%	Overall accuracy across full corpus (+1.40% vs master)
Bitrate Bias	-0.9%	Slight undershoot bias
Throughput (Avg)	+2.4% 🚀	Up to +5.5% on tonal music
Library Size	-0.26% 📉	.text: -0.38%, .rodata: +0.77%
Avg MOS Δ	-0.000	Neutral

Benchmark: https://github.com/nschimme/faac/actions/runs/33309978924