frontend: add sample-accurate gapless MP4 decoding

[nschimme]

This is a new feature to ease automated regression testing and eliminate extra padding that reference .wav files didn't have.

This PR adds sample-accurate gapless MP4 trimming via elst edit lists (with fallback to iTunSMPB metadata tags), fixes -g flag parsing, and hardens MP4 metadata atom handling.

It incorporates two critical rate-domain fixes discovered during reference testing against FFmpeg and Apple's AudioToolbox (afconvert):

• Dynamic Frame-Size Trimming: Replaces the hardcoded 1024-sample front-trim offset (net_start_trim = gapless_delay - 1024) with the actual framesize calculated from the Audio Specific Config (ASC). The fixed 1024 offset was causing 1-frame over-trimming on SBR/HE-AAC streams.
• Implicit SBR Rate Rescaling: Rescales gapless_delay and valid sample counts when container timescales match the core (pre-SBR) rate rather than the output rate. This resolves output truncation on files authored via fdkaac or afconvert.

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Verification & Proof of Safety

Trim boundaries were verified for sample accuracy by decoding test files from multiple encoders across faad2, ffmpeg, and Apple's afconvert, then measuring sample counts and RMS alignment against the raw uncompressed source:

Source Encoder	Profile	Content	faad2 Samples	ffmpeg Samples	Source Match	RMS-Optimal Shift
FAAC	LC	Synthetic + Speech/Noise	Exact match	Exact match	Exact match	0
FAAC	HE-AAC	Synthetic + Speech/Noise	Exact match	Exact match	Exact match	0
ffmpeg (native)	LC	Synthetic	Exact match	Exact match	Exact match	0
fdkaac	HE-AAC	Synthetic + Speech	Exact match (±1)*	Exact match	≈ Match	0
Apple afconvert	LC, HE-AAC	Synthetic + Speech	Exact match (±1)*	Exact match	≈ Match	0

*Matches Apple's native sample-rounding behavior.

Key Impact

• RMS Shift = 0: Confirms sample-accurate boundary trimming. Shifting decoded PCM output by ±1024 or ±2048 samples increases error by 50–80×.
• HE-AAC: Resolves the ~50% audio truncation bug on core-rate timescale files (fdkaac, afconvert) and eliminates the 1-frame over-trim on all SBR streams.
• LC Streams: Zero regression; standard AAC-LC output remains unaffected.

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Test Checklist

• Builds cleanly (cmake + ninja) on current master.
• Verified sample count and RMS alignment against ffmpeg for all encoder/profile permutations.
• Verified sample count against afconvert / aac_at for fdkaac and afconvert files.
• Validated LC output regression neutrality.