design: unify operator confirmations across reconciliation, checkpoints, accounting, and tax

[c-wri]

Why:

• balance-specific confirmations will land first so manual balance submission stops pretending user-authored rows are source-backed evidence
• the repo still needs one generic operator confirmation model so later unsupported-case handling does not fragment across reconciliation, checkpoint, accounting, and tax surfaces

What:

• define one shared operator confirmation artifact and vocabulary
• standardize attribution, timestamps, justification text, and support references
• decide which entrypoints consume shared confirmations directly versus projecting local artifacts
• connect reconciliation, checkpoint, accounting, and tax flows to the shared model
• document trust tiers and reporting requirements across those domains

Out of scope:

• replacing the balance-specific confirmation artifact in the balance PR
• weakening the source-backed checkpoint requirement for filing readiness