Add snapshot tests in CI

[senekor]

closes #1511

I confirmed that this new workflow works as expected on my fork:

• workflow is skipped if PR descrption doesn't contain magic string
• CI passes on harmless change
• CI fails if output changes

[Copilot]

Pull Request Overview

This PR adds support for snapshot tests in CI and updates documentation accordingly.

• A new GitHub Actions workflow is introduced to run snapshot tests when the PR description contains a magic string.
• The README is updated with instructions on how to trigger snapshot tests in CI and locally.
• The snapshot test in src/lib.rs now avoids a panic on directory deletion failure by swallowing the error.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
.github/workflows/snapshot_tests.yml	New workflow file for running snapshot tests based on a magic string in the PR description.
README.md	Updated instructions clarifying how to run snapshot tests in CI and locally.
src/lib.rs	Adjusted test setup to avoid panicking when the output directory removal fails.

Comments suppressed due to low confidence (2)

src/lib.rs:308

• Swallowing errors from remove_dir_all may hide underlying filesystem issues; consider explicitly handling the error and logging a warning if the removal fails.

let _ = std::fs::remove_dir_all(concat!(env("CARGO_MANIFEST_DIR"), "/site"));

.github/workflows/snapshot_tests.yml:12

• [nitpick] Using a magic string directly in the condition can reduce clarity; consider extracting 'RUN_SNAPSHOT_TESTS' into a named constant or adding a comment explaining its purpose.

if: contains(github.event.pull_request.body, 'RUN_SNAPSHOT_TESTS')