EXEC schemes and `txco://` builtins

The schemes

Scheme	Runs	Reference
http(s)://…	Your service, over HTTP	ops.md
op://NAME	Sandboxed wasm nano-op on the chassis	ops.md, sdk/op
txco://…	A chassis builtin (table below)	this page
ai://chat	A chat model via the chassis’s AI registry	ai.md
mcp+http(s)://…	A tool on an external MCP server	mcp.md
<stack>/<scope>	Unschemed stage jump (synthesized into @goto)	txcl.md

The builtin registry

Builtin	What it does
txco://noop	Returns {}. Placeholder / structural.
txco://static	Serve static files with layered lookup: the stack’s FILES/ → workspace FILES/ → embedded defaults. Caps: 1 MiB/file, 2048 files, 64 MiB total. See examples/quickstart-hello-world for the rule pattern.
txco://web-render	Read a source path, optionally render Markdown→HTML, set @web.res.*, halt. Pages without a backend.
txco://sendmail	Render + submit outbound email from the _sendmail contract — see sendmail.md.
txco://hmac-sign	Compute an HMAC signature (key via WITH secrets.*).
txco://hmac-verify	Verify an HMAC, constant-time; result lands under @computed.*.
txco://basic-auth-encode	Encode user:pass to a basic-auth header value.
txco://copy	Path-to-path copy inside the envelope (what SET can’t do with computed paths).
txco://detect-tenant	Boot-pipeline: hostname/listener → tenant resolution. Used by the scaffolded _sys/boot rules; you rarely call it directly.
txco://route	Boot-pipeline: promote a routing proposal (@route.*) into @goto + @tenant. Companion to detect-tenant.
txco://continuation-result	Poll handler behind ?_txc.continuation=<id> (continuations). Wired by the chassis; not called from rules.

Builtins pay normal fuel and appear in traces like any other op. The current registry is assembled in chassis/server/server.go; if this table and the code disagree, the code wins — and this page needs a patch.