Make it possible to muffle CRAN skips

[hadley]

This shouldn't have any effect on user code, but we need some way to ensure that CRAN skips aren't propagated across multiple test_code() boundaries so that we can properly test.

[claude]

Claude finished @hadley's task —— View job

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Code Review Complete

• Read and analyze changed files
• Check implementation for bugs/issues
• Provide review feedback

Review Findings

LGTM - The implementation correctly addresses the stated goal of preventing CRAN skips from propagating across test_code() boundaries.

Key Changes:

1. Centralized signal handling: Created signal_snapshot_on_cran() function that wraps the signal with a muffle_cran_snapshot restart
2. Added restart invocation: The snapshot_on_cran handler in test_code() now calls invokeRestart("muffle_cran_snapshot")
3. Consistent usage: Updated expect_snapshot_file() and expect_snapshot_helper() to use the new centralized function

Technical soundness:

• The restart mechanism properly contains the skip signal within test boundaries
• Function placement in R/snapshot.R is appropriate as it serves both snapshot functions
• Test change from expect_snapshot_skip() to expect_skip() correctly reflects that CRAN skips are now muffled during testing

No issues identified - The changes are minimal, focused, and implement the intended behavior without affecting user-facing functionality.