Fix Cargo cache key to include source checksum (v4)

[rileyseaburg]

Summary

Fix the CircleCI Cargo cache key to include source file checksum, ensuring Rust source changes trigger rebuilds.

Problem

The cache key only included Cargo.lock checksum. When template_renderer.rs was modified but Cargo.lock didn't change, CircleCI restored the cached (old) compiled library.

Solution

Cache key now includes both:

• Cargo.lock checksum (dependency changes)
• template_renderer.rs checksum (source code changes)

Also bumped version to v4 to guarantee a fresh build.

[Copilot]

Pull request overview

This PR addresses a caching issue in CircleCI where Rust source code changes weren't triggering rebuilds. The cache key previously only included Cargo.lock, causing the CI to use stale compiled libraries when source files changed. The solution adds template_renderer.rs checksum to the cache key and bumps the cache version from v3 to v4.

Key Changes:

• Added template_renderer.rs checksum to the Cargo cache key
• Bumped cache version from v3 to v4 to force fresh builds

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[Copilot]

The cache key only includes the checksum of template_renderer.rs, but the Rust project has multiple source files (form_data.rs, lib.rs, and template_renderer.rs). Changes to form_data.rs or lib.rs would not invalidate the cache, potentially causing stale builds. Consider using a checksum of the entire src directory or all .rs files instead of just template_renderer.rs. CircleCI supports checksumming directories, e.g., checksum "ext/widget_renderer/src" which would hash all files in the directory.

Suggested change

	- v4-cargo-{{ checksum "ext/widget_renderer/Cargo.lock" }}-{{ checksum "ext/widget_renderer/src/template_renderer.rs" }}
	- v4-cargo-{{ checksum "ext/widget_renderer/Cargo.lock" }}-{{ checksum "ext/widget_renderer/src" }}

[Copilot]

The cache key only includes the checksum of template_renderer.rs, but the Rust project has multiple source files (form_data.rs, lib.rs, and template_renderer.rs). Changes to form_data.rs or lib.rs would not invalidate the cache, potentially causing stale builds. Consider using a checksum of the entire src directory or all .rs files instead of just template_renderer.rs. CircleCI supports checksumming directories, e.g., checksum "ext/widget_renderer/src" which would hash all files in the directory.