Fix build_playground path + docs CI registry update

- build_playground: look for index.html at docs/src/playground/ (was docs/playground/)
- docs CI: use Pkg.Registry.update() + Pkg.instantiate() instead of Pkg.resolve()
  (resolve tries to upgrade deps and fails when registry isn't populated)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Dale-Black

.github/workflows/docs.yml

@@ -32,7 +32,7 @@ jobs:
         run: |
           julia --project=docs -e '
             using Pkg
-            Pkg.resolve()
+            Pkg.Registry.update()
             Pkg.instantiate()
           '
 

src/playground/build.jl

@@ -22,7 +22,7 @@ function build_playground(output_dir::String; verbose::Bool=false, bundle::Bool=
     mkpath(output_dir)
 
     playground_src = joinpath(@__DIR__)
-    docs_playground = joinpath(@__DIR__, "..", "..", "docs", "playground")
+    docs_playground = joinpath(@__DIR__, "..", "..", "docs", "src", "playground")
 
     # --- Step 1: Copy JS pipeline files ---
     js_files = ["parser.js", "lowerer.js", "infer.js", "codegen.js", "runtime.js", "worker.js"]