feat(count): add CountTracker package for finite generation

[Dylan-M]

Proposed Change

Add generator/count.Tracker — a CAS-based atomic counter for finite telemetry generation. Workers call Acquire() before each generation; when permits are exhausted, Done() closes. Reset() enables idle-mode restart via SIGUSR1.

This PR is part of a stack for PIPE-926. The full implementation plan is attached as a document on that ticket.

Checklist

• Changes are tested
• CI has passed

[Dylan-M]

• docs(count): add finite generation documentation #114 : 11 dependent PRs (#115 , #116 , #117 and 8 others)
• feat(count): wire tracker in main.go with signal handling #113
• feat(count): add count and onFinish config with overrides #112
• feat(count): add CountTracker package for finite generation #111 👈 (View in Graphite)
• main

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