feat(audio-metrics): in-memory input + agent-ready contracts

[shubhamNvidia]

Contracts: describe() + *_key params on the bandwidth, SQUIM and WER metric stages, including conditional_writes (a metric is only written when the audio resolves and estimation succeeds).

New functionality

• In-memory input (input_residency) — read the waveform already in the task rather than re-opening the file for every metric stage. On a chain of several metric stages this removes one file read per stage per row.

Per-segment scoring and metric_skip_reason already existed upstream; the contracts here just declare them, so the planner knows a metric is written only when the audio resolves and estimation succeeds.

Depends on the agent-ready foundation, #2332 — that must merge first (this branch imports nemo_curator/stages/audio/_agent/, so CI here stays red until it lands).

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[greptile-apps]

Greptile Summary

Adds agent-readable contracts and configurable output keys to the bandwidth, SQUIM, and WER metric stages.

• Adds file, waveform, and automatic input residency to bandwidth and SQUIM.
• Reuses resident waveforms with their sample rates instead of reopening audio files.
• Declares conditional top-level and per-segment metric writes for pipeline planning.
• Adds tests for waveform residency, validation behavior, and zero-length SQUIM segments.

Confidence Score: 5/5

The PR appears safe to merge after its explicitly declared agent-foundation dependency lands.

No independently actionable defect remains beyond the import failure already documented as an expected dependency-order constraint.

Important Files Changed

Filename	Overview
nemo_curator/stages/audio/metrics/bandwidth.py	Adds agent contracts, configurable data keys, and file-or-waveform audio resolution while preserving segment-level skip handling.
nemo_curator/stages/audio/metrics/squim.py	Adds batch-only agent metadata, configurable metric output, and resident-waveform support for whole-record and segment scoring.
nemo_curator/stages/audio/metrics/wer.py	Adds agent contracts and consistently applies the configurable metrics key across top-level and segment-level WER output.
tests/stages/audio/metrics/test_metrics.py	Covers bandwidth waveform parity, residency validation, SQUIM residency declarations, and zero-length segment handling.

_{Reviews (1): Last reviewed commit: "feat(audio-metrics): make metric stages ..." | Re-trigger Greptile}