Fix explicit proof version zero resolution

[DicksonWu654]

Summary

• Preserve explicitly requested proof version 0 in Foundry.resolve_proof_version.
• Distinguish an omitted version from any explicit integer value, leaving latest-version selection only for omission.
• Add a focused unit regression over the ordinary multi-version proof inventory for explicit zero, explicit nonzero, and omitted versions.

Problem

The resolver used a truthiness check for user_specified_version. Because the parser represents version 0 as integer 0, an explicit request for version 0 skipped the user-specified branch and fell through to latest-version selection. With later proof versions present, Kontrol could therefore select a different proof than requested.

Fix

Use an explicit None check so version 0 follows the same validation and selection path as every other requested version. Existing latest-version behavior remains unchanged when no version is supplied.

Regression test

The unit test covers:

• explicit version 0 returns 0
• explicit nonzero version 2 returns 2
• omitted version returns latest version 3

Validation

Tests were not run locally under the review constraint, which prohibits executing repository code and tests. Static review of the complete diff and git diff --check completed successfully.