chore(mvt): fix stale run docs (bun to deno) and declare kernel export

The README told contributors to run `bun test` / `bun run serve`, but bun is
not installed and the repo language policy bans it (Deno replaces Node/npm/bun).
The real entry points are the `deno task` definitions in deno.json, so the docs
now say `deno task test` / `deno task serve`.

deno.json gains an `exports` field pointing at the active kernel
(src/kernel/index.js), which both silences Deno's "exports field should be
specified when specifying a name" warning and declares the public entry the
README already documents as importable.

No mechanics changed; the 6 kernel tests remain green (deno task test).

Author: hyperpolymath

mvt/README.md

@@ -12,8 +12,8 @@ This directory is intentionally self-contained and dependency-free. The active r
 
 ```sh
 cd mvt
-bun test
-bun run serve
+deno task test
+deno task serve
 ```
 
 Then open `http://localhost:5173/`.

mvt/deno.json

@@ -2,6 +2,7 @@
   "$schema": "https://deno.land/x/deno/cli/schemas/config-file.v1.json",
   "name": "@hyperpolymath/econosphere-mvt",
   "version": "0.1.0",
+  "exports": "./src/kernel/index.js",
   "license": "MPL-2.0",
   "tasks": {
     "test": "deno test --allow-read tests/kernel/",