Add segment inspection tool for counts, sizes, ranges, and corruption checks

[erskingardner]

Problem

Pensieve does not currently have a lightweight way to inspect archive segment files without indexing them into ClickHouse or writing ad hoc scripts.

Researchers and operators need quick answers like: how many records are in this segment, what kinds does it contain, what created_at range does it cover, how large are records, and is the file structurally valid?

Why this matters

Before sharing or processing research archives, we need a fast sanity-check tool for segment batches. This makes archive handoff safer and debugging easier.

Suggested implementation

Add a CLI tool such as segment-inspect.

The tool should report at least:

• segment path
• gzip/uncompressed status
• compressed bytes
• decompressed bytes
• record count
• min/max/average record size
• min/max created_at
• kind histogram
• first and last event id
• structural validity status
• first parse/truncation error with byte offset, if invalid

Acceptance criteria

• Tool inspects .notepack files.
• Tool inspects .notepack.gz files.
• Tool reports record counts and byte sizes.
• Tool reports created_at range and kind histogram.
• Tool exits non-zero for malformed/truncated segments.
• Tests cover valid and malformed segment inputs.
• just precommit passes before merging.