feat(wasm): compile the kernel cube to graff-kernel.wasm

[justrach]

fx compiles to WASM as fx-core.wasm / fx-term.wasm behind a JS host (JSPI fetch, no bash). graff cannot do that for the live agent yet — std.http.Client, jobs, MCP stdio, and TTY restore do not compile to wasm32-freestanding.

What does compile is the maths: the ToolCatalog cube ({0,1}^6 → 64 name lists) and the lexical path jail. This PR ships that as the first artifact.

What you get

zig build wasm
# zig-out/bin/graff-kernel.wasm  (~2.8 KB, ReleaseSmall)

import { loadGraffKernel } from "./sdk/wasm/graff-kernel.js";
const k = await loadGraffKernel("./graff-kernel.wasm");
k.catalog({ lean: true });          // 8 names
k.advertised("subagent", { isSub: true }); // false
k.confined("/etc/passwd");          // false

No JSPI. No WASI. Not the agent.

Guardrails

• Native tests replay the same fixtures Lean exports (spec/kernels/tool_catalog.json, path_confine.json).
• scripts/eval-tier1.sh --only wasm is compile-only so the target cannot rot.
• CI runs zig build wasm on Linux and Windows.
• ADR 0012: a later graff-core.wasm / graff-term.wasm needs an explicit host capability table (fetch, workspace, no implicit bash), the same way fx does. Do not compile src/main.zig and hope.

Until that host layer exists, embed the live harness with graff serve + the remote SDK.

Demo after zig build wasm: python3 -m http.server 8080 → /sdk/wasm/demo.html.

CI

sdk typecheck failed on process.off("unhandledRejection") — bun-types only overloads Process.off for memoryPressure. Fixed by casting through EventEmitter in remote.test.ts (also on #593).