feat: add Std.Broadcast type

[algebraic-dev]

This PR adds a multi-consumer, multi-producer channel to Std.Sync.

This PR depends on: #10366, #10367 and #10370.

[leanprover-community-bot]

Mathlib CI status (docs):

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[leanprover-bot]

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• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase eabd7309b74f13cc12d0ab03d4826f34e3e49c0d --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-09-22 22:57:29)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 055060990c6ac614cb8ba210b241272fe563c73e --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-09-25 00:01:16)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 5ef7b45afaedd794fb39b0fbd70924b573694a00 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-09-25 13:17:36)
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[hargoniX]

Looks generally like a sound approach, just some comments/questions regarding styling and invariants.