Type: typed map<K,V> (vs Composite struct / Json)

[rrrodzilla]

Workstream: Type system · Part of #89

Context

CEL has a typed map<K,V>. SchemaForge has Composite (fixed-key struct, BTreeMap<String, DynamicValue> over a declared field set) and Json (untyped) — but no typed, open-keyed, homogeneous map. CEL map comprehensions (m.all(k,v,…), m.exists(...)) are only useful with a real map type.

Scope

• New FieldType::Map { key, value } + matching DynamicValue.
• DSL syntax; distinguish clearly from Composite and Json in docs.

Acceptance

• A typed map field round-trips; rule comprehensions over it type-check.

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Dependencies: Type is parallelizable now; map comprehensions are exercised once the evaluator (#91) lands. Soft.

[rrrodzilla]

Landed in 551dfba — feat(types): add typed map<K,V> field type (#99).

End-to-end typed, open-keyed, homogeneous-value map, distinct from Composite (fixed declared field set) and Json (untyped):

• core: FieldType::Map { key: Box<FieldType>, value: Box<FieldType> } + new DynamicValue::Map(BTreeMap<String, DynamicValue>) (distinct from Composite).
• DSL: map<K, V> syntax (map keyword + </>); recursive key/value parse.
• cel: bridge DynamicValue::Map → CelValue::Map (string keys, recursing values) so map comprehensions (m.all(k,v,…), m.exists(...)) type-check (Integration: apply-time type-checking of rule expressions against schema field types #104 → CelType::Map) and evaluate.
• storage: Postgres JSONB; SurrealDB native object (with FLEXIBLE for open keys). The Type: duration (records retention / TTL / SLA) + timestamp arithmetic #96/Type: bytes (inline binary — hashes, signatures, key material) #97 fail-closed write checks now recurse into Map values too.
• acton REST: JSON object both directions; every value validated against the homogeneous V (fail-closed → 422 on mismatch).

Key-type decision: constrained to string (map<string, V>) for this cut — JSON/JSONB/SurrealDB objects are all string-keyed, so non-string keys can't round-trip without lossy encoding. A non-string key is rejected at DSL parse and at the API (DslError::MapKeyNotString / 422) with an actionable message. The { key, value } structure is forward-compatible; non-string keys are a noted follow-up. Map-vs-Composite-vs-Json distinction documented in docs/site-guide.md.

Verified: clippy -D warnings clean; full workspace 2123 tests pass under --features surrealdb.