feat(model): simulate models from rules over information links

Phase 2 brings Models to life. ADR-0004: behaviour comes from a small fixed
vocabulary of rules (Constant / Proportional / Gap) read over the inbound
Information Links, not free-form formulas — valid by construction.

- types: Rule union, SimSpec, initialValue; replaces the unused equation field
- simulation: forward-Euler engine, dependency-ordered evaluation, algebraic-loop
  detection, non-negative stocks, and a divergence guard
- io: validate and round-trip the new fields (F8)

README.md

@@ -19,6 +19,9 @@ Deployed at https://meadows.apoena.dev
     substrate; the domain Model is the source of truth.
   - [ADR-0003](./docs/adr/0003-flow-as-node-materialised-clouds.md) — a Flow is a
     node; Source/Sink clouds are materialised nodes.
+  - [ADR-0004](./docs/adr/0004-rate-rules-not-formulas.md) — simulation behaviour
+    comes from a small fixed vocabulary of rules over Information Links, not
+    free-form formulas.
 
 ## Stack