Separate diff telemetry from semantic changes

[Widthdom]

Summary

• classify database diff changes as data, schema, readiness/provenance, or volatile telemetry with stable reasons
• make semantic comparison ignore volatile run/FTS maintenance telemetry by default, add --data-only and --include-telemetry, and bind comparison modes to cursors
• preserve bounded summary behavior, keep reasons mode-independent, register generated completions, and update bilingual docs/tests/changelog

Root cause

diff mixed volatile index-run metadata with semantic identity and did not expose a stable categorized reason for every non-identical result.

Validation

• Debug and Release builds: 0 warnings, 0 errors
• final Release diff + flag-schema tests: 70/70 on net8.0 and 70/70 on net9.0
• Codex review reruns: diff tests 48/48 on net8.0 and net9.0; export/import tests 67/67 on net8.0 and net9.0
• cross-contract tests (console/import/JSON): 345 passed on net8.0
• full Release net8.0: 10,870 passed, 1 known flaky watch failure, 7 skipped; the failure reproduces on origin/main
• full Release net9.0: 10,397 passed, 0 failed, 420 skipped
• generated completion check, dotnet format --verify-no-changes, changelog fragment validation, git diff --check, and local index freshness passed
• Codex adversarial review (2 rounds): all 6 findings fixed. Post-fix diff audit: No blocking/actionable issues found.

The known validation flake is tracked in #4955 and #4966.

Documentation and changelog

• updated USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md in their matching English/Japanese sections
• added changelog.d/unreleased/4884.fixed.md

Fixes #4884