feat(engine): config groups — one definition fans out routing + strategy to N models

[hugocorreia90]

What

The declarative shape of the governed-materialization fan-out (the C4 keystone). A models/groups/<name>.toml definition is opted into by N models via group = "<name>", supplying shared routing (schema_template, filled per model from its [args]) and strategy — so a one-line change flows to every model in the group.

# models/groups/daily_marts.toml
schema_template = "mart_{region}"
[strategy]
type = "merge"
unique_key = ["id"]
update_columns = ["amount", "status"]

# models/fct_orders.toml
group = "daily_marts"
[target]
catalog = "warehouse"     # schema comes from the group template
[args]
region = "emea"           # {region} -> schema "mart_emea"

Design

• Precedence: per-model sidecar > group > _defaults.toml. A model can still pin its own schema/strategy; the group overrides directory defaults.
• Zero ModelConfig churn: group fields live on RawModelConfig; loading rides a side-loader (load_groups_from_dir); resolution threads through new *_with_groups variants of load_model_pair/parse_model_inline. The bare functions delegate with None, so all existing callers are untouched.
• One template grammar: schema_template reuses the schema-pattern resolve_template ({name} / {name:SEP}).
• Wired through both compiler loaders: .sql via load_models_from_dir, and .rocky DSL via the salsa project loader — verified end-to-end through rocky emit-sql (compile::compile) for each, with a .rocky regression test so the DSL path can't silently drop groups.

Guardrails (load-time, deliberately small/neutral)

• Unknown group reference → ModelError::UnknownGroup.
• schema_template placeholder the model doesn't supply → ModelError::InvalidGroup (no routing to a literal mart_{region}).
• Unknown key in a group file → rejected (deny_unknown_fields).

Tests

rocky-core: group supplies strategy + routes schema; sidecar overrides group; group overrides _defaults.toml; unknown group errors; missing template arg errors. rocky-compiler: .rocky model resolves a group through the salsa loader.

Scope / follow-ups

v1 carries schema_template + strategy. Shared tags (→ Dagster asset metadata) and per-mode computed keys, plus compile-time governance invariants (enforcement: detect a grouped model that overrides its routing), are follow-ups. The group reference persists in each sidecar so enforcement has a hook.

cargo fmt + clippy --all-targets clean; no codegen/schema drift.