011 trellis variant guard

// TEST (aspirational): std/trellis:guard — CAPABILITY-FLOAT early warning.
//
// The categorization payoff: variant selection consults trellis
// membership. guard binds a variant tag to a trellis; any flow that
// REQUESTS that variant (|gpu selector) but does not satisfy the trellis
// dies at the request site with a located, koru-level message — early
// warning instead of generated-code wreckage three stages later.
//
// Desired message shape (pinned as substring in expected_error.txt):
//
//   flow requests variant "gpu" but does not satisfy trellis "gpu-legal": std/kernel:step is not gpu-legal
//
// LADDER NOTE (each rung is its own red reason, in order):
//   1. std/trellis:guard does not exist yet.
//   2. The parser SILENTLY SWALLOWS the |gpu selector on a transform
//      invocation (pinned as 210_140) — until that carries, the guard
//      has nothing to read.
//   3. Membership annotations (rung 2 of the categorization engine)
//      carry the per-flow verdict the guard consumes.
//
// SURFACE NOTE (caps ruling, 2026-07-03): Lars ruled that mechanism
// vocabulary must not leak into annotations — capability requirements are
// declared as `[cap("...")]` intent annotations that trellis REASONS
// OVER, not as trellis(...)/guard bindings. The guard-invocation spelling
// below is a placeholder; the binding design for transform-invocation
// variants (no proc to annotate) is open — see
// baton_trellis_capability_annotations. The MESSAGE shape is the stable
// part of this pin.

~import std/trellis
~import std/kernel
~import std/io

~std/trellis:define("gpu-legal")
| `std/kernel:init/kernel/std/kernel:step.*` _ => error "std/kernel:step is not gpu-legal (sequential outer iteration)"
| `.*` _ => ok

~std/trellis:guard(variant: "gpu", trellis: "gpu-legal")

~std/kernel:shape(Body) {
    mass: f64,
}

// VIOLATION: requests |gpu but uses step — fails the gpu-legal trellis.
~std/kernel:init(Body)|gpu {
    { mass: 1.0 },
}
| kernel k |> std/kernel:step(0..2)
    | step |> std/kernel:self { k.mass += 0.5 }
| computed c |> std/io:print.blk {
        b={{ c[0].mass:f }}
    }