add test package.json with postinstall canary script

[pullfrog]

Adds a package.json written during a dependency installation behavior test. The postinstall script writes a canary file to /tmp/postinstall-canary.txt to verify whether npm lifecycle scripts run during install.

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Note

Low Risk
Test-only manifest change; postinstall writes to /tmp and does not affect production app code.

Overview
Replaces the previous minimal package.json (vitest test script) with a test-pkg manifest whose postinstall script writes CANARY_MARKER to /tmp/postinstall-canary.txt.

This supports dependency-install behavior tests that confirm whether npm lifecycle scripts actually run during install.

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