Document why setup-arcium was removed and add caching TODO

Added comments explaining:
- The arcup 0.6.2 bootstrap bug that prevents using setup-arcium action
- TODO to restore setup-arcium once the bug is fixed
- TODO to add caching for Rust, Solana CLI, Anchor, and Arcium to reduce
  CI build times

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mikemaccana

.github/workflows/test.yml

@@ -9,6 +9,10 @@ on:
 jobs:
   run-arcium-build:
     runs-on: ubuntu-latest
+    # TODO: Add caching for Rust, Solana CLI, Anchor, and Arcium installations
+    # to reduce CI build times. Consider using actions/cache for:
+    # - ~/.cargo (Rust toolchain and Anchor)
+    # - ~/.local/share/solana (Solana CLI)
     steps:
       - uses: actions/checkout@v3
 
@@ -28,6 +32,11 @@ jobs:
       - name: Install Anchor
         run: cargo install --git https://github.com/solana-foundation/anchor --tag v0.32.1 anchor-cli --locked
 
+      # TODO: Restore setup-arcium action once arcup 0.6.2 bootstrap bug is fixed
+      # Currently cannot use arcium-hq/setup-arcium action because arcup 0.6.2 has a bootstrap bug
+      # where it panics with "index not found" at arcup/src/config.rs:29:51 when running
+      # 'arcup list' or 'arcup install'. The arcup binary doesn't have version 0.6.2 in its
+      # internal configuration/version list. Workaround: download arcium CLI binary directly.
       - name: Install Arcium CLI
         run: |
           curl "https://bin.arcium.com/download/arcium_x86_64_linux_0.6.2" -o ~/.cargo/bin/arcium