refactor(engine): extract hardcoded Dutch legal domain knowledge

[ehotting]

Context

The engine's design principle is "zero built-in domain knowledge" — everything should come from the law YAML files. However, several pieces of Dutch legal domain knowledge have leaked into the engine code. This issue tracks their identification and extraction.

Real domain knowledge (should extract)

#	What	Where	Description
1	Eurocent rounding	service.rs:1349	Engine treats type_spec.unit == "eurocent" as special: rounds float outputs to integer. Should be a generic TypeSpec mechanism where rounding behavior is declared in YAML, not hardcoded per unit name.
2	Gemeente_code scoping	resolver.rs:285	Hardcoded field name gemeente_code for territorial filtering. Comment says "add provincie_code here later". Should be a generic scope field system driven by law metadata.
3	Regulatory layer hierarchy	priority.rs:23-47	Full Dutch constitutional hierarchy hardcoded as rank ordering: Verdrag(0) > EuVerordening(1) > Grondwet(3) > Wet(4) > AMvB(6) > MinisterieleRegeling(7) > GemeentelijkeVerordening(9) > Beleidsregel(10) etc. 12 layers with hardcoded ranks used for lex superior resolution.
4	Delegation type validation	service.rs:1460	Engine validates that an implementing regulation's regulatory_layer matches the open term's delegation_type. Knows which layers can delegate to which.

Borderline (discuss)

#	What	Where	Notes
5	Legal character / AWB procedures	article.rs:114, resolver	The Produces struct and hook/procedure resolution framework. Values are strings from YAML, but the framework itself encodes Dutch administrative law concepts.
6	Date arithmetic clamping	operations.rs:855	Jan 31 + 1 month = Feb 28. Cites HR ruling on termijnberekening. Standard calendar math but documented with Dutch legal reference.

Not domain knowledge (keep in engine)

#	What	Why keep
7	Age calculation (operations.rs:834)	Standard whole-years definition
8	Day of week (operations.rs:1010)	ISO 8601
9	Float→int safety (operations.rs:1147)	Numerical correctness
10	Precision limits (operations.rs:26)	Numerical correctness

Approach

Each extraction should be done as a separate PR. Possible approaches per item:

1. Eurocent → generic type_spec rounding/coercion mechanism declared in YAML
2. Gemeente_code → generic scope field system where laws declare their own scope dimensions
3. Layer hierarchy → externalize rank table as configuration (or keep as engine knowledge with strong justification)
4. Delegation type → derive validation rules from the hierarchy configuration

References

• RFC-003: Inversion of Control
• RFC-013: Execution Provenance
• How It Works — concepts — states the zero-domain-knowledge principle

Supersedes #361.

[ehotting]

Proposed approach: engine policy as configuration YAML

Instead of extracting domain knowledge into code-level configuration, express it as a policy YAML file that the engine reads at startup. This is not part of the corpus — it is engine configuration, but expressed in a structured, auditable format.

Layered configuration (IoC pattern)

1. National default — ships with the engine distribution. Defines the Dutch legal system baseline:

   • Regulatory layer hierarchy and rank ordering
   • Scope field definitions (gemeente_code, provincie_code, etc.)
   • TypeSpec coercion rules (e.g., eurocent → round to integer)
   • Delegation validation rules

2. Org-level override — each organization can override specific policy rules for their context. Same mechanism the engine already uses for laws (higher level sets defaults, lower level can override).

Why this works

• Engine becomes truly zero-domain-knowledge — even its operating rules are externalized
• Policy is versioned, auditable, and part of the Execution Receipt (RFC-013)
• A different jurisdiction (or a test harness) can swap in a different policy file
• Organizations with special requirements can override without forking the engine
• Connects to RFC-013 §5 (compliance requirements as machine-readable regulations)
• Uses the same layered override pattern the engine already implements for law execution