docs: add OpenAPI definition for the sequencer HTTP API

[tuler]

What changed

Adds openapi.yaml at the repo root: an OpenAPI 3.1 definition of the 14 routes the sequencer serves, derived from the Axum handlers and serde wire types and cross-checked against the README API section.

• ingress (public): POST /tx, GET /fee, GET /nonce, GET /domain
• history (internal): GET /ws/subscribe, GET /history, GET /historical-l1-inputs
• snapshots (internal): GET /finalized_state/inclusion_block, GET /finalized_state, GET /latest_snapshot, GET /finalized_snapshot
• health (internal): GET /livez, GET /readyz, GET /healthz

No code or behavior changes.

Why

A machine-readable description of the API for client generation, documentation tooling, and app developers integrating against the sequencer.

Risk / compatibility notes

• Second copy of the contract. The README API section remains the owner; the spec says so in its description. Nothing links to the file or checks it in CI yet, so it will drift unless kept in sync.
• Not linted. The YAML parses and every $ref resolves, but it has not been run through an OpenAPI linter (e.g. Redocly or Spectral).
• WebSocket frames. OpenAPI cannot describe messages after the upgrade. /ws/subscribe documents the handshake and its refusals; the frame shape is the BroadcastTxMessage schema, referenced through a non-standard x-websocket-messages extension.
• Non-JSON errors are modelled as such. /ws/subscribe returns a plain-text 400 for bad query fields, and the snapshot routes return empty-body 404/500, rather than the {ok, code, message} shape.
• u64 fields are typed integer with the full u64 maximum, which exceeds strict format: int64; some generators may complain or pick a signed type.

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