Add the data spine: typed records, query engine, and Memory/gRPC proxies

[tannevaled]

What

The headless data spine of the go-widgets ecosystem — the model layer an application skeleton binds its grids and lists to. Pure Go, CGO=0; the core imports only the standard library and go-widgets/mvvm.

Core (package data)

• Value / Kind — a comparable typed scalar (string / int / float / bool).
• Record / Schema / Field / Rule — a typed row and its validation (Required, StrMinLen/StrMaxLen, NumMin/NumMax), mirroring toolkit's validation shape lifted to typed values.
• Query → Apply → View — a pure query engine: filter (eq/ne/lt/le/gt/ge/contains), multi-key sort (asc/desc), group-by, paginate (rows or groups), aggregate (count/sum/avg/min/max, grand + per-group).
• Proxy seam (List / Query / Mutate) with MemoryProxy, the in-process reference backend.
• Store[R] — a typed, bindable collection wired through mvvm.ObservableList, so a view re-renders on data change.
• Canonical(View) — a deterministic, map-order-independent encoding used to compare views byte for byte.

Transport (package grpcproxy)

• A small list/query/mutate .proto service (datapb, generated) carried over grpc-transports/websocket. The Client is a data.Proxy that compiles to js/wasm (a browser speaks the same gRPC service); the Server is native-only (//go:build !js).

Proxy conformance — the key test

grpcproxy runs a battery of 15 sort→filter→group→page→aggregate queries through the MemoryProxy, the remote GRPCProxy, and the engine directly, canonicalises every resulting View, and requires all three identical byte for byte:

proxy conformance: 15 queries, MemoryProxy == GRPCProxy == engine (byte-identical)

So the same data code yields identical results native and over the wire (browser/wasm) — proven with a live gRPC-over-websocket round trip, not asserted.

Quality

• 100% statement coverage on data/ and grpcproxy/ (the generated datapb/ protobuf excepted — filtered from the gate, DO NOT EDIT).
• gofmt / vet clean, -race clean.
• Cross-compiles all six 64-bit Go targets + js/wasm + darwin + windows (the wasm build proves the GRPCProxy client is browser-ready).

Notes for the reviewer

• New repo under the go-widgets org; not auto-merged — opened green for review.
• The data spine is a separate module rather than living inside go-widgets/toolkit: the toolkit is stdlib-only (just painter), and pulling gRPC + the websocket transport into it would pollute that purity and churn a repo that is co-edited live. This mirrors the go-widgets/mvvm precedent.
• Org-conformance landing / logo / favicon / docs are a follow-up.

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