Add /base subpath export and document Jest setup

[Ryang-21]

What

Adds a ./base subpath export to the package, so offline primitives can be imported without the rest of the SDK:

import { StrKey, Keypair } from "@stellar/stellar-sdk/base";

Also adds base/index as an explicit rollup entry (guaranteeing emission independent of tree-shaking, like http-client), documents the new subpath in the stellar-base migration section, and adds a "Usage with Jest" README section covering the transformIgnorePatterns recipe and the ≥es2020 compilation-target requirement for the ESM-only crypto dependencies.

Why

Since the v16 stellar-base fold-in, the package root is the only way to reach primitives like StrKey. The root is a barrel, and under CommonJS require() can't tree-shake it, so importing one symbol loads Horizon, RPC, and the SEP helpers along with their networking dependencies (axios, eventsource, smol-toml) — which breaks Jest consumers on ESM-only transitive deps. With /base, a CJS require loads 101 modules with zero networking/SEP modules, versus 174 for the root import. The remaining Jest friction (@noble/* being ESM-only) is a test-runner transform concern, so it's documented rather than worked around in the build.

Closes #1533

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[Shaptic]

changelog entry pls