Allow candidate Card after-image preparation

[KimGLee]

Summary

• allow explicit write-time stamp_cards --set-version to prepare a candidate Standards Card after-image while canonical state still names the before version
• keep ordinary --check bound to canonical active state and reject candidate-version check overrides
• document the preparation/adoption boundary, remeasure K00/03, and regenerate affected Card/interface artifacts
• add an end-to-end regression covering candidate preparation, pre-adoption staleness, and post-adoption verification semantics

Why

The state/history separation in #124 correctly made .cambium/governance/standards_state.yaml the current identity. Its first adopter migration then exposed a cycle: the adoption transaction requires complete candidate Card bytes in its after snapshot, but the Card compiler refused to produce those bytes until canonical state already named the candidate version.

Preparing a candidate is not adopting it. This change separates those phases while keeping adopt_standards as the only current-state writer.

Verification

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest Tools.tests.test_stamp_cards_commands (19 passed)
• make check
• git diff --check

Fixes #125