feat: tagged checkpoints

[namn-grg]

Close #21

[julio4]

Great, thank you for this contribution!
I would just like to change how we handle tag in different checkpoint instances, but else this looks good to me!

[Copilot]

Pull Request Overview

This PR introduces tagged checkpoint functionality, replacing the previous named barrier system with a more flexible tagging mechanism. The changes allow checkpoints to be tagged with metadata for easier reference and identification in history queries.

• Replaces named barriers with a flexible tagging system on checkpoints
• Adds new methods for querying checkpoint history by tags
• Updates display and debug output to include tag information

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/test_utils/step.rs	Updates test utilities to use tagged barriers instead of named barriers
src/payload/ext/checkpoint.rs	Adds new methods for querying checkpoint history by tags
src/payload/checkpoint.rs	Implements the core tagging functionality and removes named barrier mutation

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[karim-agha]

I'm not sure if we should allow changing any kind of state on an existing checkpoint.

In my opinion tags (if we chose to go with this design) should be attached during checkpoint creation and remain immutable for the whole duration of checkpoints lifetime and the lifetime of all its clones.

[julio4]

I'm not sure if we should allow changing any kind of state on an existing checkpoint.

In my opinion tags (if we chose to go with this design) should be attached during checkpoint creation and remain immutable for the whole duration of checkpoints lifetime and the lifetime of all its clones.

Hm I agree. Lets make checkpoint immutable by nature. Then we could overload apply/barrier with a _with_tag

[namn-grg]

I'm not sure if we should allow changing any kind of state on an existing checkpoint.

In my opinion tags (if we chose to go with this design) should be attached during checkpoint creation and remain immutable for the whole duration of checkpoints lifetime and the lifetime of all its clones.

Makes sense, I agree.

• moved the tag field to inner
• added _with helpers
• removed the tag value changing fns (set, clear)