Add Arch source package core harness

[MaxGhenis]

Summary

Splits the production source-package core out of the broader source-package parity branch (#3) so Arch can land the stable framework without the large raw-source/package expansion.

This PR adds:

• canonical aggregate fact/source-cell/source-row models and validation
• declarative source-package loading and build-suite harnesses
• consumer-contract JSONL export, SQLite DB build, bundle, mirror, and artifact helpers
• two fixture-backed SOI source packages (soi-table-1-1, soi-table-1-4) using existing small raw fixtures
• docs for repository/storage architecture, source-package authoring, and the target-construction harness

It intentionally does not include the broad package catalog or new raw publisher artifacts from #3. Those should land in follow-up source-package PRs after this core harness is reviewed.

Verification

• uv run ruff check arch db tests/test_arch_*.py tests/test_supabase_client.py
• uv run pytest tests/test_arch_core.py tests/test_arch_harness.py tests/test_arch_database.py tests/test_arch_mirror.py tests/test_arch_suite.py tests/test_arch_source_package.py tests/test_arch_soi_targets.py tests/test_arch_artifacts.py tests/test_arch_boundaries.py tests/test_arch_bundle.py tests/test_arch_concepts.py tests/test_arch_consumer_contract.py tests/test_arch_pe_source_plan.py tests/test_arch_source_cells.py tests/test_arch_source_rows.py tests/test_supabase_client.py -q
• uv run pytest -q

Full suite result: 607 passed, 16 skipped.