feat(pxq): Stage A skeleton — PXF → gojq → PXF (loose mode)

[trendvidia]

Summary

Implementation Stage A for pxq, the jq-style query tool whose
design was landed in #30 / #31. End-to-end spine — read a PXF file,
run a jq query, emit PXF — in loose mode with PXF as the only
input adapter. JSON/YAML/CSV adapters, strict mode, the @pxf.*
extension namespace, the @proto(...) constructor, and the
infer-schema subcommand all land in follow-up stages.

Why bundled this way

The README design covers a broad surface (4 input formats, 2 modes,
schema chain, gojq integration). A single end-to-end PR would be too
large to review carefully. This stage demonstrates the minimum
interesting demo — a jq query against a real PXF document with
@dataset rows working — so the moving parts are exercised together
before any one part grows.

What's here

File	Purpose
cmd/pxq/main.go	cobra entrypoint; positional <query> <file> args
cmd/pxq/load.go	pxf.Parse → gojq's untyped any graph (loose-mode rules per README)
cmd/pxq/run.go	gojq embedding; loose-mode errors-as-null with stderr hints
cmd/pxq/emit.go	untyped graph → PXF text (schema-less best-effort)
cmd/pxq/main_test.go	11 tests, including the README's quick-start filter example

Loose-mode rules (from the README)

• INT → int64 if it fits, else *big.Int
• FLOAT → float64
• BYTES → base64 string with "b" prefix (round-trips through emit as b"...")
• @type / @dataset / @proto / @<name> → exposed on synthetic
  __pxf_* top-level keys for raw jq access; Stage C swaps these for
  the @pxf.directive(name) extension surface
• Runtime errors degrade to null (jq-compatible)

Demo

$ cat trades.pxf
@dataset trades.v1.Trade ( symbol, price, qty )
( "AAPL", 188.42, 100 )
( "MSFT", 415.10,  50 )
( "AAPL", 188.55,  75 )

$ pxq '.__pxf_datasets[0].rows | map(select(.symbol == "AAPL")) | length' trades.pxf
value = 2

Once Stage C lands, the __pxf_datasets synthetic key becomes
@pxf.directive("dataset") — the README's documented form.

Dependencies

• protowire-go 0.75.0 → 1.0.0 (consumes the v1.0 AST surface:
  Document.Datasets / .Protos / .Directives)
• New: github.com/itchyny/gojq v0.12.19

Test plan

• go build ./... clean
• go test ./... — 11 new cmd/pxq tests pass; existing
  internal/pxfschema tests unaffected
• Smoke test against the README's quick-start example
• CI passes on the matrix