feat: execute an isolated Bambu Studio slice

[kforris]

Objective

Execute one explicitly requested Bambu Studio slice in an isolated run directory and emit evidence that is distinct from a plan or physical-print claim.

Acceptance criteria

• Invoke Bambu Studio with an argument array and shell=False.
• Require a fixture with documented redistribution permission and a pinned source commit.
• Record slicer version, input SHA-256, duration, exit status, output hashes, and a redacted log.
• Prevent credentials, serials, local usernames, and absolute home paths from entering the manifest.
• Add red/green tests with zero skip/xfail and keep branch coverage at or above 85%.
• Pass every validation command in AGENTS.md.

Out of scope

Cloud login, printer control, network plugins, model upload, and physical-print claims.

[kforris]

Implementation is ready for review in #2.

Verified evidence:

• explicitly approved, isolated Bambu Studio 02.06.00.51 execution;
• project-authored CC0 fixture pinned to commit 2d2c54e9d9ddc53a365606b9d9d070b94caeb64c;
• two successful same-input/settings runs with exit 0 and matching G-code SHA-256;
• portable manifest records version, input/profile hashes, duration, exit status, output hashes, redacted log hash, and physical_validation: null;
• local validation: 45 passed, 0 skipped/xfail, 87.38% branch coverage, lint/format/mypy/build/diff checks green;
• PR CI is green on Python 3.11, 3.12, and 3.13: https://github.com/kforris/OpenPrintBench/actions/runs/30368347793

Repeatability record: https://github.com/kforris/OpenPrintBench/blob/817e8a7c8fb7c21264fead9229d277d3da55b6e2/docs/evidence/2026-07-28-bambu-stl-repeatability.md

This is digital slicing evidence only. No physical validation is claimed. The issue remains open until the PR is merged and the resulting main CI is green.