Praxis mode 1/8: Add the Rust workspace and port the inference parser #1718

Part of #1714 · phase A, delivery step 1 · Section 3 · design in #1717 (docs/superpowers/specs/2026-11-07-praxis-mode-design.md)

Goal

Start the Rust side of praxis mode: a workspace outside go.work, and the first parser port, with a gate that keeps Go and Rust in step.

Scope

• A new Rust workspace praxis/ with the crate cortex-praxis-filters. It is outside go.work, so the 11 Go modules, go vet and go mod tidy -diff do not change.
• The parser module cortex_parse for inference: a port of core/parse/inference. It is a module, not a filter: Praxis mode 2/8: Build cortex-gateway and the Cortex ingestion endpoint #1719 wires it into the one filter cortex_observe, because Praxis runs response hooks in reverse chain order.
• It parses the request buffered, and SSE responses frame by frame, including the known mislabeled SSE on a 2xx only (a port of isKnownMislabeledSSE with its 2xx gate, server.go:926-933).
• It decodes gzip and zstd for parsing only, and counts decoded bytes against the 10 MB buffered cap and the 10 MB per-frame cap, as direct mode does. The rows over a cap are in the design, Section 3.
• The parity gate: a Go test writes the expected event JSON next to each Turn the MCP, A2A and inference parsers into libraries, with saved real-traffic test cases #1503 fixture, and a Rust test must give the same JSON. It blocks PRs under praxis/. On a Go-only parser PR it reports and does not block.
• The schema drift test: a Go test writes the SessionEvent wire field list. A Rust test fails when a field is neither set nor listed as "not set in praxis mode".
• A Rust CI job that runs only on praxis/**, with a size-capped cache. A Go-only PR does not pay for Rust.

Before work starts

• Decided 2026-11-09 (Tracker: Move the laptop data plane to Praxis when the upstream gaps close #1714, D6): Cortex owns the Rust parsers. Another owner takes them later (U15). praxis-ai publishes about 10 of the about 35 inference fields and keeps no parsed body by design, so a port to praxis-ai is not a drop-in.

Done when

• Every inference fixture from Turn the MCP, A2A and inference parsers into libraries, with saved real-traffic test cases #1503 gives the same event JSON in Go and in Rust.
• A Go-only PR runs no Rust job.
• The crate uses only HttpFilter, register_filters!, emit_deferred_record and filter metadata from the Praxis API.

Depends on

Nothing.

Assisted-By: Claude (Anthropic AI) noreply@anthropic.com