Deferred hardening follow-ups from PR #30 review

[nijoe1]

Tracking the explicitly-deferred items from the PR #30 follow-up review (2026-07-23). None block the merge; each is bounded.

• Check-then-stream TOCTOU (upload/multi-upload): a path can change between stat()/preflight and streaming. Open an fd up front and fstat/stream from it so the validated file is the streamed file.
• Fresh-address RPC matching (wallet balance): failed to apply on state is broad and could misclassify another RPC failure as ADDRESS_NOT_ON_CHAIN. Tighten (e.g. confirm actor state explicitly via a cheap read) instead of substring matching.
• wallet init existing-keystore no-op path: without an explicit method, only an existing private key is detected as already_configured — an existing keystore falls through to the interactive prompt. Also make --source mutation atomic with a successful init (today it can persist before a later failure).
• Docs fetch centralization: move the host allowlist inside docsFetch() itself so every future call site is covered by construction; optionally reject non-default ports.
• Retrieval wording: distinguish "bytes matched the CID at retrieval time" from ongoing PDP proof status in download's description, README, and skills before release.

Context: PR #30 follow-up review, "Safe to defer" section.