feat(files): add editable artifact versions

[roxi3906]

Summary

Adds immutable editing and version comparison for allowlisted UTF-8 text Artifacts and Uploads. Markdown, plain text, scripts, and source-code files can be edited as raw text; each save publishes a new managed version while preserving source lineage.

Related TAPD parent: 1137438753001002003.

Highlights

• Adds header Edit and Compare actions, raw-source editing, version navigation, and predecessor diff highlighting for editable text files.
• Stores each edit as an immutable version named v<8 lowercase alphanumeric>_<original name> and records current head, base version, original file metadata, storage identity, checksum, and provenance in SQLite.
• Resolves current DB heads for preview, download, project/session export, mentions, Agent inputs, and references while preserving explicit historical-version reads.
• Supports generated Artifacts and uploaded files, including generated same-run predecessor chains, legacy recovery, and the no-native Windows export fallback with size/checksum verification.

Impact

• Adds immutable application migration 0004_managed_file_version_foundation; released migrations 0001-0003 remain unchanged.
• Expands the native managed-file module with anchored read/write/verify primitives and adds diff@9.0.0.
• Touches Main, preload, shared contracts, renderer preview state, project catalog/reference resolution, and Electron E2E.
• Keeps non-text preview behavior unchanged and gates editing through the centralized UTF-8 allowlist/capability result.

Test Results

All checks ran after the final material edit and integration rebase.

• Full portable suite: npx vitest run --maxWorkers=4 - 1,021 files and 15,195 tests passed; 14 files / 208 tests skipped.
• Build and type safety: npm run build - Node/Web typecheck and Electron/Web production builds passed.
• Lint: npm run lint -- --no-cache - 0 errors; 10 existing warnings outside this change.
• Real Electron workspace journey: npx playwright test e2e/workspace-files.spec.ts - 2/2 passed, covering Markdown edit/version/diff navigation and existing image preview.
• Database schema/ledger: npm run db:schema:check and focused migration/ledger suites passed.
• Independent module reviews, rebase-integration reviews, and final standards/spec reviews: Ready: Yes.

Potential Issues

• Local real-Electron validation ran on macOS; cross-platform native builds and the Windows fallback remain authoritative in PR CI.
• The Windows fallback is covered by unit/integration tests, but a real Windows UI journey was not run locally.
• The feature crosses multiple existing file consumers; review should focus on migration/current-head semantics, atomic publication, exact historical reads, and preview edit/diff state.

Authors

• Roxi (@roxi3906)