Harden injection lifecycle and transport boundaries

[smiggleworth]

Summary

• preserve reusable caller-owned Request bodies and explain genuinely consumed inputs
• make dispatch abortable even when a target never settles
• wrap malformed URL and method construction with actionable package context
• cover repeated, redirected, concurrent, failure, cleanup, and public-export scenarios
• document exact fidelity and divergence from the Node transport

Validation

• npm run check
• 28 runtime tests
• 100% line and function coverage; 89.47% branch coverage
• public type contract
• publint and artifact checks
• isolated JavaScript and TypeScript package-consumer smoke tests
• package version remains 0.2.0

Closes #24
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Closes #30

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