Add common pipeline timestamp

[CyGei]

Addresses issue #3 by adding a common timestamp to all steps within the same pipeline in the log file.
Adds a vignette illustrating usage.
Updates get_log() and rm_log() to support retrieving or removing all logs.

[Copilot]

Pull Request Overview

This PR adds a common timestamp to all steps within the same pipeline in the log file. The key enhancement is that all time_pipe() calls within the same pipeline now share a common start timestamp, making it easier to correlate timing data across pipeline steps.

• Added pipeline-level timestamp tracking to ensure consistent timing baselines
• Enhanced get_log() and rm_log() functions to support retrieving/removing all logs at once
• Added comprehensive vignettes demonstrating usage patterns and comparisons

Reviewed Changes

Copilot reviewed 15 out of 23 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
R/time_pipe.R	Core logic updated to track pipeline start times and maintain consistent timestamps
R/get_log.R	Enhanced to support retrieving all logs when log = NULL
R/rm_log.R	Enhanced to support removing all logs with force = TRUE parameter
R/emit_time.R	Updated function signature to accept pre-calculated duration and start time
R/zzz.R	Added start_times tracking list to the pipetime environment
vignettes/*.Rmd	Added new comprehensive vignette and updated existing one
man/*.Rd	Updated documentation to reflect new functionality
tests/testthat/test-time_pipe.R	Consolidated and improved test coverage
README.Rmd/.md	Updated examples and documentation

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[Copilot]

Missing unit validation. The function should validate that unit is one of the accepted values before passing it to difftime() to prevent runtime errors.