feat(storage): isolate datasets by project

[tuzuminami]

Summary

Closes #40.

• Add schema v3 projects, explicit opaque UUID scope, legacy backfill, foreign-key constraints, and per-project revision/CAS protection.
• Carry project context through the desktop selector, Tauri allowlist, local API, recording, import, analysis, export, and deletion paths.
• Prevent recording/switch/delete races and avoid re-running migrations for each scoped request.
• Update the non-engineer workflow and project-boundary documentation.

Legacy migration

Existing data is atomically backfilled into the deterministic Migrated data project. Migration records before/after counts and content fingerprints.

Validation

• ./scripts/test.sh (124 passed)
• ./scripts/lint.sh
• ./scripts/check_licenses.sh
• ./scripts/check_no_external_network.sh
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml (29 passed)
• ./scripts/perf_smoke.sh

Residual scope

Workspace backup/restore and all-data deletion policy remain owned by #52 and #54; clearing a selected project does not delete workspace migration snapshots.