Explain thrown exceptions versus Err, callbacks, and freezing limits

[0thernet]

Summary

• README.md:121-127 distinguishes recoverable Err values from throwing callbacks, explains fallback limits and lack of runtime freezing; verified src/index.ts:13-35.
• Part of a portfolio documentation pass that applies Stripe and Vercel docs patterns: a complete first result, task-based navigation, exact defaults, and troubleshooting checked against source.

Test plan

• bun test ./src/readme.test.ts: 5 passed, 0 failed
• host-run --mode=shared --lane=compute --label=docs-result-final -- bun run check: Exit 0 after waiting 988.5 seconds; typecheck, build, package consumer checks, 11 tests passed, 0 failed.
• Changed commands, defaults, and claims checked against the owning source; independent review findings addressed
• Required CI passes on this head

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