perf: reuse Conda history while parsing environments (Fixes #475) (#493)

## Summary

- read `conda-meta/history` once per environment parse
- reuse the snapshot for manager discovery, Python package metadata, and
environment naming
- preserve package JSON fallback when history is absent or incomplete
- add regression coverage that verifies one history-reader invocation

## Validation

- `cargo test -p pet-conda`
- `.\scripts\rust-precommit.ps1`

Fixes #475

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: karthiknadig