Add AAC-ELD support (ELD frame lengths, tables, MDCT/FFT/TNS updates)

[skrashevich]

Motivation

• Add decoding support for AAC-ELD streams which use 512/480-based frame lengths and ELD-specific signaling and tables.
• Reuse existing decoder pipeline and extend components to handle ELD frame-size variants, scalefactor bands and TNS limits.

Description

• Parse AAC-ELD AudioSpecificConfig (AOT_ER_AAC_ELD / profile 39) in pkg/decoder/decoder.go and set FrameLength to 512 or 480, including basic ELD extension skipping and SBR detection.
• Add ELD scalefactor band offsets and sample-rate (7350 Hz) entries and window-band counts in pkg/tables/tables.go, and expose SWBOffset512/SWBOffset480 and corresponding band-count arrays.
• Make ICS/TNS frame-length-aware: pkg/ics uses FrameLength to pick SWB tables and disallow short-window sequences on unsupported lengths, and pkg/tns now builds TNS with frame-length-aware maxBands for 1024/512/480.
• Make filterbank configurable for arbitrary supported frame lengths via NewWithFrameLength in pkg/filterbank and generate matching long/short windows dynamically.
• Extend MDCT (pkg/mdct) and FFT (pkg/fft) to support ELD-related transform sizes (1024/128 and 960/120 via generated twiddle tables and additional FFT lengths) by generating sine/cos tables at runtime when precomputed tables are not present.
• Update unit tests and test helpers to reflect new supported FFT/MDCT sizes and expanded table dimensions.

Testing

• Ran the full test suite with go test ./... and all package tests passed successfully (unit tests for fft, mdct, filterbank, ics, tns, tables, decoder, etc.).
• Ran gofmt on modified files as part of local validation prior to testing.

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Add FFT support for 30/32 or skip short MDCT init

Allowing MDCT lengths 128/120 for ELD means m.N4 becomes 32/30, but fft.New still only supports lengths 64, 512, 256, 60, 480, and 240 (see pkg/fft/fft.go around the switch in New). As a result, mdct.New(128|120) fails at fft.New(m.N4), and filterbank.NewWithFrameLength always builds the short MDCT, so AAC‑ELD 512/480 frame lengths will error during decoder initialization even though short windows are disallowed. This blocks all ELD decoding unless you add FFT support for 32/30 or avoid constructing the short MDCT for those frame lengths.

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