feat: arche-synthetic supplier world

[denironyx]

What this changes

data/synthetic/ generates 2,000 suppliers observed through a stale ERP, a fresh registry and OCR'd invoices, and labels every resulting disagreement change / representation / error with its cause. 1,189 pairs differ because the entity legitimately changed, the column no public ER benchmark has, and the one ledger.observe() needs to be measurable at all.

Checklist

• uv run pytest packages/arche-core/tests -q passes
• uv run ruff check packages/arche-core/src is clean
• Tests cover the change, and a bug fix has a test that fails without it
• CHANGELOG updated if this is user-visible
• Docs updated if this changes documented behaviour
• No real personal data in code, tests, fixtures, commit messages, or
  this description

If this touches documentation

• Every code snippet was executed, and the pasted output is what it
  actually printed
• mkdocs build --strict passes
• Any number claimed is reproducible from something in this repository

If this touches resolution, policy, or signing

• Abstention is preserved — this does not turn a review into a match by
  lowering the bar for calling two records the same entity
• Any new statute mapping cites its specific section
• Anything that enters a signature is deterministic across runs
• Breaking changes to signed output or verification are called out
  explicitly above

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