feat(deno): migrate echidna-playground npm -> Deno (standards#253) (#155)

## Summary

Class B (npm-wrapper via Deno) for `echidna-playground/` sub-tree per
standards#253 STEP 3 (smallest-first batch).

- Deletes redundant `echidna-playground/package.json`
(rescript-toolchain stub only — 0 runtime deps, 1 rescript devDep)
- Updates `echidna-playground/deno.json` tasks: `npx rescript` / bare
`rescript` -> `deno run -A --node-modules-dir=auto npm:rescript`
- Adds `.gitignore` entries for npm-avoidant lockfiles
(package-lock.json, bun.lockb, yarn.lock, pnpm-lock.yaml, .npmrc)

`deno.json`'s `imports` already carried the `rescript: ^12.0.0`
specifier (resolves via npm), so no new dependency declaration needed.

## Test plan

- [ ] CI green
- [ ] `deno task res:build` works locally (post-merge spot-check)

Refs: standards#253 STEP 3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.gitignore

@@ -217,3 +217,11 @@ generated/tlaiser/
 generated/wokelangiser/
 generated/alloyiser/
 generated/abi/
+
+# npm-avoidant (standards#67)
+package-lock.json
+**/package-lock.json
+bun.lockb
+yarn.lock
+pnpm-lock.yaml
+.npmrc

echidna-playground/deno.json

@@ -2,13 +2,13 @@
   "name": "@coq-jr/web",
   "version": "0.1.0",
   "tasks": {
-    "build:res": "npx rescript build",
-    "clean:res": "npx rescript clean",
+    "build:res": "deno run -A --node-modules-dir=auto npm:rescript build",
+    "clean:res": "deno run -A --node-modules-dir=auto npm:rescript clean",
     "dev": "deno run --allow-net --allow-read server.ts",
     "serve": "deno run --allow-net --allow-read server.ts",
-    "res:build": "rescript build",
-    "res:clean": "rescript clean",
-    "res:watch": "rescript build -w"
+    "res:build": "deno run -A --node-modules-dir=auto npm:rescript build",
+    "res:clean": "deno run -A --node-modules-dir=auto npm:rescript clean",
+    "res:watch": "deno run -A --node-modules-dir=auto npm:rescript build -w"
   },
   "imports": {
     "@std/http": "jsr:@std/http@^1.0.0",