[Feat] xazz-stdlib crate

[ax1s-x1zz]

Context

docs/ROADMAP.md Track B-2. Reusable helpers should be versioned with the workspace instead of copy-pasted.

Scope

• New workspace member with date/string/statistics helpers exposed as .xzz-visible operators
• Workspace-versioned, docs generated, demo usage

Acceptance

import "std/math" usable in a demo pipeline.

Dependencies

Depends on B-1 (module system).

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See docs/ROADMAP.md for the full scale plan and execution order.