Add resumable partial-family member continuation

[Widthdom]

Summary

• add authoritative total, returned, omitted, and remaining counts plus recovery/next cursors to grouped partial-symbol families while retaining the 50-member materialization cap
• page family members in SQL with deterministic ordering and a fixed-size family identity, binding cursors to the normalized selection and index generation and rejecting malformed, mismatched, stale, or mid-read generation changes
• preserve the recovery contract across array, NDJSON, compact, byte-bounded, and explicit-field projections without loading a complete large family into managed memory

Validation

• dotnet restore CodeIndex.sln
• dotnet format CodeIndex.sln --no-restore
• dotnet build CodeIndex.sln -c Release --no-restore (0 warnings, 0 errors)
• full Release tests: net8.0 — 11,727 passed, 7 skipped; net9.0 — 11,248 passed, 423 skipped; 0 failed
• targeted Issue Provide resumable continuation for truncated partial-family members #5101 test on net8.0 and net9.0, including a 105-member family with an over-4,096-character identity
• related partial-family suite: 32 passed on net8.0
• dogfood traversal of the repository's 105-member SymbolExtractor family as 50/50/5 members, plus explicit --fields path recovery-metadata verification
• root status --check reports index_matches_workspace: true; workspace manifest reports both members healthy
• mandatory Codex adversarial review completed; all actionable findings were addressed and regression-tested

Documentation and changelog

• updated the English and Japanese sections of USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md
• added changelog.d/unreleased/5101.added.md

Follow-up candidates

• None.

Fixes #5101