[codex] Use DPM and validate Java

[cgdusek]

Summary

• switch the CLI compile and test subcommands from the deprecated daml executable to dpm
• validate Java by running java -version instead of trusting any java binary on PATH
• document DPM and JDK 17+ as explicit requirements

Why

Issue #1 reproduces in a current DPM-based environment:

• create-canton-app compile and create-canton-app test fail because they still hard-require daml
• the create flow prints Java Runtime found on macOS even when only the /usr/bin/java launcher stub is present

Impact

• current Canton/DPM users can run the shipped subcommands without installing the legacy Daml CLI
• macOS users get an actionable warning when Java is not actually usable for dpm test

Validation

• env PATH="$HOME/.dpm/bin:$HOME/n/bin:/usr/bin:/bin:/usr/sbin:/sbin" node /Users/charlesdusek/Code/create-canton-app/bin/create-canton-app.js stub-check --template Multiparty
• env PATH="$HOME/.dpm/bin:$HOME/n/bin:/usr/bin:/bin:/usr/sbin:/sbin" node /Users/charlesdusek/Code/create-canton-app/bin/create-canton-app.js compile
• env PATH="$HOME/.dpm/bin:$HOME/n/bin:/usr/bin:/bin:/usr/sbin:/sbin" node /Users/charlesdusek/Code/create-canton-app/bin/create-canton-app.js test
• env PATH="$HOME/.dpm/bin:/opt/homebrew/opt/openjdk@17/bin:$HOME/n/bin:/usr/bin:/bin:/usr/sbin:/sbin" JAVA_HOME="/opt/homebrew/opt/openjdk@17/libexec/openjdk.jdk/Contents/Home" node /Users/charlesdusek/Code/create-canton-app/bin/create-canton-app.js test

Closes #1.