chore: fix span cleanup in Namespace

[famouswizard]

Description

spans weren’t closing properly, now they do.

• _guard keeps spans alive until dropped.
• removed id, dispatch.exit(), and mem::forget hacks.
• Namespace::new just takes a name, spans enter immediately.
• ns! macro simplified, no more manual span juggling.

cleaned up, simpler, and spans now end correctly.

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• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer