feat(engine): config-group misplacement guard — reject incoherent args + pinned schema (C5)

[hugocorreia90]

What

The buildable remainder of C5. Most of C5 — "a model's target schema derives from its location/metadata" — was already delivered by C4: config groups derive a member's schema from schema_template + per-model [args], and enforce = true hard-blocks an override for enforced groups. This PR adds the one piece that was missing: the misplacement guard for the incoherent config the plan calls "a misplaced model fails compile."

A group's [args] exist solely to fill its schema_template. If a member also pins its own target.schema, the pin wins (sidecar > group), so the template is bypassed and the args are dead. That's never a valid override — it usually masks a routing mistake ("I thought [args] routed this model, but the pin sends it elsewhere"). It now fails the load:

# group marts: schema_template = "mart_{region}"
group = "marts"
[target]
schema = "escape"     # pins schema → bypasses the template
[args]
region = "emea"       # ...so this can never route anything → ERROR

ModelError::GroupArgsWithPinnedSchema. Pin a schema or supply args, not both.

How

• Detected in the shared resolve_model_config chokepoint, next to the existing enforce check — so it covers both .sql and .rocky for free (.rocky regression included). No ModelConfig churn, no new warning channel; a load error fails compile, the same path GroupOverride already uses.
• Group-schema resolution is now lazy: a model that pins its own schema skips the template entirely. Without this, a legitimate pin-without-args override would error on an unfilled {region} placeholder for a template it never uses. ([args] is template-only — verified no other consumer — and unreleased, so no existing project changes behavior.)

Allowed cases (unchanged)

• Pin a schema, no args → override (template bypassed).
• Supply args, no pin → route via template.
• Enforced group + pin → already GroupOverride.

Only the contradiction (group with a template + args + pin) is rejected. Two existing tests that encoded the now-forbidden args+pin pattern were updated to the coherent override (pin, no args).

Verification

cargo test --all-features -p rocky-core -p rocky-compiler green (incl. the guard, the allowed companion, and the .rocky regression); cargo fmt --check + clippy --all-targets --all-features clean.

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