frontend: add iTunes gapless playback metadata (iTunSMPB)

[nschimme]

This PR adds support for iTunes-compatible gapless playback metadata (iTunSMPB) in MP4 container outputs and exposes exact encoder delay numbers via the public API.

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End-User Impact

• Seamless Track Transitions: Fixes micro-pauses, silent gaps, and clicks between songs when playing live albums, DJ mixes, or continuous concept albums on Apple Music, iTunes, and iOS devices.
• Exact Audio Length: AAC encoders naturally introduce extra silent samples (priming delay and trailing padding) to pad frames. Media players will now read the iTunSMPB tag and skip those extra samples, ensuring audio track lengths match original source files down to the exact millisecond.
• Correct HE-AAC Playback: Correctly accounts for complex internal buffering in HE-AAC audio streams so low-bitrate files won't cut off early or glitch on playback.

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Technical Breakdown

1. Exposes Encoder Delay via Public API (faac.h / libfaac)

   • Adds encoder_delay to faac_encoder_info to report priming delay in samples/channel at the output rate.
   • Calculates delay per profile (faacEncoderDelay):
     • LC-AAC: 1 frame of MDCT overlap (FRAME_LEN).
     • HE-AAC: SBR pipeline buffering minus resampler group delay (3 * FRAME_LEN - RESAMPLE_FILTER_LEN / 2).
   • Note on ABI: Adding encoder_delay to faac_encoder_info changes the struct size. Since we are already bumping the ABI version in this release, including this public API addition now is harmless and avoids needing another ABI bump later.

2. Frontend Gapless Calculation (frontend/encode_engine.c)

   • Calculates exact priming delay and padding samples using input vs. output sample totals.
   • Passes gapless parameters directly to mp4_set_gapless().

3. iTunes iTunSMPB Metadata (frontend/mp4write.c)

   • Formats and writes standard iTunSMPB hex strings under the com.apple.iTunes freeform atom in ilst.----.
   • Fixes put_tag_ext(): Corrects freeform atom structure to write a single data child atom inside ilst.---- for spec compliance.

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Verification

• Delay calculations verified against decoded output via impulse testing across various sample rates, bitrates, and channel configurations (LC-AAC and HE-AAC).
• Benchmark https://github.com/nschimme/faac/actions/runs/32757060121?pr=387