stereo: dynamic IS crossover and MS restriction at low bitrates

[nschimme]

This is a quality improvement.

Passes core bandwidth into AACstereo() to improve low-bitrate encoding quality. Dynamically scales Intensity Stereo (IS) crossover and disables short-window Mid/Side (M/S) stereo under constrained bandwidths to eliminate pre-echo artifacts and save bit allocation for low-frequency phase detail.

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Benchmark Data & MOS Results

• Benchmark Results & MOS Artifact Analysis: https://github.com/nschimme/faac/actions/runs/33200780571

Key MOS Highlights

• Significant Low-Bitrate Lift: Major MOS gains at constrained stereo bitrates ($\le$48 kbps):
  • 24k Stereo: +0.592 ABR (49/0 wins) | +0.614 VBR (49/0 wins)
  • 32k Stereo: +0.396 ABR (49/0 wins) | +0.363 VBR (46/3 wins)
  • 40k Stereo: +0.213 ABR (43/2 wins) | +0.238 VBR (42/4 wins)
  • 48k Stereo: +0.105 ABR (40/4 wins) | +0.214 VBR (40/7 wins)
• High Bitrate Neutrality: Zero regression at > 128 kbps (mono tracks and high-bitrate stereo maintain identical baseline performance).

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Technical Changes

• Disable Short-Window M/S at Low Bitrates: Prevents transient pre-echo when bandWidth <= 16000 Hz in JOINT_MIXED mode by falling back to JOINT_IS.
• Dynamic IS Crossover: Scales crossover frequency between 3.5 kHz (IS_START_FREQ_MIN) and 7.0 kHz (IS_START_FREQ_MAX) based on bandWidth * 0.35 (capped at 0.35 * Fs).
• Loop & Memory Optimizations: Precalculates IS target bin offsets (target_offset_long / target_offset_short) outside per-element loops and replaces element-wise clearing loops with memset() in apply_ms() and apply_mute().
• API Update: Updates AACstereo() signature in libfaac/stereo.h / libfaac/stereo.c to take unsigned int bandWidth and updates the call site in libfaac/frame.c.