Populate and validate the local LCSC jlcparts cache

[ReidSurmeier]

Context

The LCSC client is now offline by default and refresh is explicit. Split jlcparts archives were downloaded during an earlier test run, but extraction could not complete because 7z was unavailable.

Outcome

Complete the explicit refresh on the current workstation and verify a known C-number lookup without exposing provider or host secrets.

Acceptance criteria

• install or provide a supported split-archive extractor
• reuse valid existing archive parts rather than downloading them again
• create the local SQLite database through the explicit refresh path
• verify schema integrity and at least one known lookup
• update INSTALL.md with dated current-host evidence