feat(asap-common): RQE expression tree type and validation #880

Part of the RQE frontend tracker (linked below once filed). Decisions live in the tracker.

What

A shared serde type for an RQE expression tree in asap-common/dependencies/rs/asap_types, used by the planner (--planner saved) and the query engine.

{"op": "/", "lhs": {"rqe": {...}}, "rhs": {"rqe": {...}}}

Node kinds (tagged enum):

• rqe: a self-contained leaf: id, metric (a table name for SQL), capability, lookback_ms, grouping (labels), optional phi (quantile), optional k (top-k), optional value (SQL value column).
• agg: {"agg": "avg|min|max|sum", "by": [labels], "of": <node>}
• binary: {"op": "+|-|*|/", "lhs": <node>, "rhs": <node>}
• scalar: {"scalar": <f64>}

Validation (one function, called at load)

• leaf capability is one of sum, quantile, cardinality, topk_by_value (others rejected: rate, topk_by_count, …)
• phi present iff quantile; k present iff top-k
• agg.by is a subset of the child's output labels
• binary: two non-scalar sides must have the same output label set; at least one side is non-scalar
• a top-k leaf is only allowed at the root

Also: a helper returning the tree's leaves, and one returning a node's output labels.

Done when

• type + validation + unit tests (each rule has a rejecting test; round-trip JSON tests for every node kind)