Pin the Heroku build's NodeJS version to what the app requires

[tricknotes]

Follow-up to #670, which documented this requirement. This makes the generator satisfy it.

Why

The package.json generated for Heroku carried only cacheDirectories. Heroku's NodeJS buildpack reads that file — the project root's, not the Ember application's — to decide which NodeJS to build with, and falls back to the current LTS release when it names no engines.node.

The requirement belongs to the Ember application: the ember-cli >= 6.8 blueprint needs NodeJS >= 20.19.0 (ember-new-output v7.0.0). Nothing carried that up to the file Heroku actually consults, so rake ember:compile could run on a NodeJS the application rejects.

What changed

• EmberCli::PathSet#package_json — the Ember application's package.json.
• EmberCli::App#node_engine — its engines.node, or nil when the file is missing or unreadable. The callers ask in order to describe the application to a build host, and a project without a readable package.json is still deployable, so it answers nil rather than raising.
• HerokuGenerator reads engines.node from each configured application and writes the one they agree on into the generated package.json.

Applications that disagree are reported through say_status and left unpinned rather than resolved: one build serves all of them, and picking a single answer would pin the build to a version the others reject.

Verification

$ bin/rspec spec/lib spec/generators
148 examples, 1 failure

The one failure is EmberCli::App#test exits with exit status of 0, which fails on main in this container too — ember test reports Launcher Chrome not found, unrelated to this change.

New coverage:

• App#node_engine — declared, absent engines, missing file, malformed JSON.
• The generator's engines key — applications agreeing, none declaring, applications disagreeing.

Also confirmed end-to-end against the installed dummy application: EmberCli["my-app"].node_engine returns ">= 20.19.0".

Rebased onto #673

This branch was rebased after #673 merged. Besides the CHANGELOG.md conflict, the generator spec's instance_double(EmberCli::App, …) still stubbed yarn_enabled?, which #673 removed — three examples failed until it was updated to yarn?. Git could not see that one.

🤖 Generated with Claude Code

https://claude.ai/code/session_014rAfkAVGVifbaEoTVeT66u