docs: ADR 0003 — enterprise/regulated scope with layered model

[robcohen]

Summary

Proposes adopting enterprise/regulated-grade accounting as the scope of pta-standards, via a four-layer specification architecture that preserves the current file-format spec unchanged at Layer 1 and adds semantic, reporting, and regulatory layers above it.

What this changes (if accepted)

• Adopts a layered architecture: concrete syntax (current) → semantic model → reporting → regulatory output
• Commits to referencing external standards (ISO 4217, IAS/ASC, XBRL, SAF-T) rather than reinventing accounting concepts
• Extends conformance levels to reflect which layers an implementation targets (L1-L4)
• Sketches an 8-phase roadmap for implementing the vision over ~2-3 years

What this does NOT change

• Layer 1 (current beancount v3 / ledger v1 / hledger v1 specs) is preserved unchanged. Existing files remain valid.
• No breaking changes to the current format.
• PTA-community users can ignore Layers 2-4 entirely.

Why now

The current spec's "UNDEFINED: pending clarification" items are almost all questions that would be answered by referencing a standard (GAAP, IFRS, IAS, ASC). Answering them ad-hoc recreates work that accounting standards bodies have already done. Referencing those standards resolves the questions and makes the spec meaningful to accountants.

Open questions

The ADR explicitly leaves 6 questions unresolved for follow-up RFCs. Key ones:

• Chart of Accounts authority (publish, reference, or agnostic?)
• Backward-compatibility strategy for unstructured Assets:Bank:Checking-style accounts
• Which reference implementation leads Layer 2+
• Relationship to a potential beancount v4

Review request

This is an architectural proposal, not an implementation PR. Looking for:

• Feedback on the layered architecture
• Pushback on scope creep concerns
• Volunteers for Phase 1 (semantic model scaffolding)
• Professional accountants willing to review Layer 2+ semantics

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