Rewrite on-disk order/invoice/bom.json after Executor execution

[JEJodesty]

order.json/invoice.json/bom.json under data/input/data/output are only ever written before
Executor.execute() runs (by Service.initBOMcar()/getEnhancedBom()), never rewritten after.
Their order_cid value matches the final Invoice's, but the files still only reflect the bootstrap-stage BOM/Invoice/log shape (e.g. log_cid: null) rather than the fully executed one.

Related: the following can where the post-execution write-back is added

• Bounded CAT Output: Node outputs BOM with Invoice & Logs #156 (Bounded CAT Output)
• BOM Composer #152 (BOM Composer)

AC:

• After Executor.execute()/Service.execute() return, the final bom/invoice/order are written
  back to their on-disk paths (data/output/bom.json, data/output/invoice.json,
  data/input/order.json), overwriting the bootstrap-stage versions
• docs/DEMO.md/docs/TEST.md are updated if the on-disk file semantics change as a result