Fix loading manifests from stdin (closes #810)

[deekshaNVIDIA]

Summary

Closes #810.

Piping a manifest into a lhotse CLI command that takes - for stdin (the original report used lhotse split) failed with orjson.JSONDecodeError: unexpected character: line 1 column 1 (char 0):

gunzip -c data/librispeech/raw_cuts_dev-clean.jsonl.gz \
  | lhotse split -s 16 - data/librispeech/tmp

Root cause

load_manifest_lazy_or_eager("-") routes to load_manifest_lazy, which:

1. Reads one line of stdin to detect the manifest class.
2. Returns a LazyManifestIterator that opens the input again on every iteration.

But stdin is a one-shot stream — it cannot be re-opened or seeked. When the CLI later materialises the iterator (e.g. in split_sequence), the second pass reads either truncated or empty data and raises JSONDecodeError.

Fix

lhotse/serialization.py

• When path == "-", eagerly slurp stdin into a list of dicts and dispatch to the same from_dicts candidate-class logic as load_manifest. This was extracted into a small _load_manifest_from_stdin helper to keep load_manifest_lazy strict (lazy means lazy).

lhotse/bin/modes/manipulation.py

• lhotse split and lhotse split-lazy derived output filenames from the input path's stem/suffix. With manifest = Path("-") that produced out/-.0 (no extension) and tripped store_manifest's "Unknown serialization format" check. They now fall back to manifest.<idx>.jsonl.gz when the input is -, so the user's command works end-to-end.

Verification

test/test_load_manifest_stdin.py (new) — covers the stdin path with monkeypatched sys.stdin:

• full eager load returns the right items in order
• split(num_splits=4) succeeds and preserves all items (the original #810 regression)
• the manifest can be iterated multiple times
• type auto-detection works (RecordingSet vs SupervisionSet)
• empty input returns None
• explicit manifest_cls= is honoured
• garbage input raises an error

I also ran an end-to-end CLI test that mirrors the issue's gunzip | lhotse split - invocation: 8 dummy recordings → manifest.{0..3}.jsonl.gz with all recordings preserved.

black, isort --profile black, and flake8 --select=E9,F63,F7,F82 are clean on the changed files. The local test suite has unrelated Windows-only NamedTemporaryFile PermissionError failures (present on master too) — CI on Linux is unaffected.