feat(control_plane): BackendPlan wire types

[zzylol]

Summary

Implements §3 of #433 (control_plane/docs/design-backend-plan-wire-format.md): the BackendPlan/Materialization/RoutingEntry Rust types, a proto schema (proto/backend_plan.proto), and round-trip conversions between them.

• Reuses this deployment's canonical vocabulary directly rather than re-encoding it: asap_sketch::{SummaryKind, SummaryParams} for the materialization payload (one wire shape for both exact and approximate families — no separate ExactAggregate variant, per the design doc §3), asap_ir/control_plane::intent_algebra's Source/ColumnRef/WindowKind for the L3 IR fragments, control_plane::sketch_algebra::capability::Capability for routing, and asap_types::{PolicyFingerprint, MonitorSpec} (already shared with data_plane for exactly this cross-crate reason).
• StorageBackend/RetentionPolicy are new, deployment-local types — control_plane cannot depend on data_plane's existing StorageBackend (dependency runs the other way), so this mirrors its variants rather than reusing it directly, matching the same constraint asap_types::MonitorSpec's own doc comment already documents for that type.
• Added one derive (PartialEq on asap_types::MonitorSpec) to make round-trip test equality checks possible — no behavior change.

Scope: types + round-trip tests only, no behavior change. Not wired into emit/, main.rs's planning path, or any data_plane consumer — RoutingIndex (design doc §4), which is what actually reads this at query time, is separate work.

Test plan

• cargo build --release -p control_plane — clean
• Round-trip tests in control_plane/src/backend_plan/mod.rs: every SummaryKind/SummaryParams variant, every Capability variant, every StorageBackend variant, Source/ColumnRef, and a full BackendPlan with both an exact and an approximate materialization — all encode/decode to equal values. Also covers the missing-oneof decode-error path (no panics on malformed input).

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