feat: structured RPC route

[0xpolarzero]

Warning

This PR is part of a stacked PR chain.

Upstream merge PRs:
wevm/incur#144 ->
wevm/incur#145 ->
wevm/incur#143 ->
wevm/incur#147

Correctly based review stack in this fork:
#3 ->
#4 ->
#5 ->
#2

Overview

Adds a structured RPC endpoint for command execution. This is the transport base for the TypeScript client (implemented in wevm/incur#143).

API

POST /_incur/rpc
content-type: application/json

{
  "command": "project deploy",
  "args": { "id": "p1" },
  "options": { "dryRun": true }
}

Changes

• Add POST /_incur/rpc for structured command calls.
• Add structured command execution so RPC receives args and options separately.
• Return validation fieldErrors from HTTP command responses.
• Reject raw fetch gateways from structured RPC with FETCH_GATEWAY_UNSUPPORTED; callers should use generated OpenAPI commands or direct HTTP routes.
• Document the RPC endpoint in the README.
• Add a changeset.

Tests

• Adds RPC route tests for execution, shape validation, schema validation errors, and raw fetch gateway rejection.
• Adds structured parse-mode validation coverage.