test: complete fetch runtime audits

[smiggleworth]

Closes #27.
Closes #28.
Closes #29.
Closes #30.
Closes #31.

Summary

• prove retry counters, deadlines, content variants, and middleware attempts remain isolated across 20 synchronized concurrent calls
• add direct round-trip coverage for every runtime codec and missing client/DSL exports
• expand replay coverage to DELETE bodies, exact deadline boundaries, and mixed/nested null query values
• correct the README Node runtime claim to match the Node 24 engine contract
• re-verify documented middleware ordering through the existing executable README-order test

Cross-package verification

• rebuilt current clean askr-schema and askr-server mains locally
• passed real askr-schema.safeParse request/response codec round-trip
• passed an actual askr-server application success response (200 application/json)
• passed server validation-error decoding (422 application/problem+json, source-prefixed issues path)
• no request deduplication or in-flight sharing implementation exists to audit
• no codec size limit is declared, so there is no maximum-size boundary

Validation

• npm run check (32 tests plus types, build, installed smoke, publint, pack inspection)
• cross-package runtime probe against rebuilt sibling packages

No package version or publishing changes.

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