Promote alloc_mem to a documented public API

[ChrisRackauckas-Claude]

Ignore until reviewed by @ChrisRackauckas.

Summary

• declare the existing alloc_mem workspace hook public through SciMLPublic
• document its prototype, opaque-cache contract, extension rule, return value, and executable usage
• render it under the workspace API and test the generic fallback and cache-consuming path
• bump the package minor version because this adds public API

Motivation

alloc_mem is already the allocation hook paired with exponential! and is used by downstream solver packages, but it was not declared public. Downstreams should be able to rely on the owner-declared API instead of carrying ExplicitImports exceptions.

Validation

• Julia 1.10 Pkg.test(): 741 passed, 1 pre-existing broken
• Julia 1.10 package QA: 19/19 passed; extension QA: 1/1 passed
• Julia release package QA: 21/21 passed; extension QA: 1/1 passed
• Julia 1.10 Documenter build passed, including doctests
• targeted public API tests passed on Julia 1.10 and release
• Runic 1.7.0 and git diff --check passed