Support the current jlcparts cache schema

[ReidSurmeier]

Closes #17

Summary

• support both the current jlc_components schema and the legacy split-table layout
• parse current range-price strings alongside legacy JSON tiers
• validate required columns and fail clearly on unknown schema drift
• enforce query-only SQLite connections
• record dated current-workstation cache evidence
• ignore both directory and symlink forms of .venv

TDD evidence

• current-schema and malformed-schema fixtures failed before implementation
• focused LCSC contract: 4 passed
• full suite: 149 passed, 2 known fixture skips, 1 documented SKiDL xfail
• compileall, shell syntax, and diff checks passed
• 1,000 live price strings sampled: 1,000 parsed, 0 failures
• live cache: SQLite quick check ok, C8734 lookup found, exact-MPN search returned the requested three results
• Gitleaks history and proposed-tree scans found no leaks
• GitNexus reports low upstream risk across the sourcing, design, MCP, and verify import paths