docs(wiki): copy and expand wiki with SDP, requirements, and test plan Port the design pages from the zehrer-xyz wiki and add planning/process docs: an SDP, numbered requirements (REQ-01..REQ-55), and a requirement-to-test matrix. Document the now-implemented String Store v1 API and on-disk format, add an Architecture page, and refresh the sidebar. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>