fix(indexer): classify storage read failures

[randomblocker]

Closes #79.

What changed

• classify storage reads as STORAGE_NOT_FOUND, STORAGE_NOT_PUBLIC, STORAGE_RPC_UNAVAILABLE, or STORAGE_INVALID_RESPONSE
• stop retrying stable missing/private responses while retaining bounded retries for transient and malformed responses
• expose only fixed public-safe diagnostic messages and keep upstream bodies, exceptions, URLs, and stack traces private
• advance scan state to v8 so existing generic failures are re-derived from chain history
• document that a Demos not-found response is operational evidence only and never authoritative DACS-5 BB-8 absence evidence

Why

The indexer collapsed every storage read failure into STORAGE_UNREADABLE. Operators could not distinguish nonexistent locators from private programs or temporary node failures, and stable conditions consumed pointless retries.

Impact

Directory operators receive actionable cause counts and per-locator guidance without weakening DACS-5 fail-closed semantics. Existing deployments replay history once to reclassify the generic rows.

Validation

• npm run typecheck
• 176 non-publisher tests passed
• 2 publisher tests passed separately
• npm run build