Rename find_canonical_root -> find_uqps_root locator API

[sherryzyh]

Summary

Renames the prkit project-locator helper for the renamed consumer project (formerly canonical_answer_protocol, now uqps / uncertainty_quantification_via_physics_semantics).

• find_canonical_root → find_uqps_root (and its __all__ entry + the find_repo_root dispatcher)
• env var PRKIT_CANONICAL_ROOT → PRKIT_UQPS_ROOT
• hardcoded sibling-directory lookups repointed from canonical_answer_protocol → uncertainty_quantification_via_physics_semantics
• sampling_backfill.py import/call + fallback path updated
• doc reference updated

Unrelated domain uses of "canonical" (unit canonicalization, canonical_text, the --canonical-sampling-root baseline flag) are intentionally left untouched.

Why

The consumer project was renamed to align with the NeurIPS draft "Uncertainty Quantification for Open-Ended LLM Physics Reasoning via Physics Semantics." The old locator hardcoded a directory name that no longer exists.

Verification

• pytest: 1378 passed, 1 skipped (incl. tests/prkit/core/test_project_env.py)
• pre-commit (ruff/black/mypy) green
• Cross-repo: find_uqps_root() resolves to the renamed sibling directory

🤖 Generated with Claude Code

[sherryzyh]

Superseded by #4. The decoupling removes the renamed locator (find_uqps_root) entirely, so this standalone rename no longer makes sense on its own — it's folded into #4 as the first commit. Closing in favor of #4.