docs(plans): error-variants audit — 6-PR breakdown of ~80 sites (#583)

Fresh audit against main (post-#582) of v3-api-ergonomics §5 item 1:
parent estimated ~42 untyped Error::Simple/Message sites; actual is ~80
across 27 files (helpers in common/error_helpers.rs and decoder paths
in realtime/accounts/contracts/news/historical weren't in original tally).

plans/error-variants-audit.md categorizes every site by target typed
variant and slices the sweep into 6 small PRs:
  PR-1 validation → InvalidArgument
  PR-2 EOF / no response → UnexpectedEndOfStream
  PR-3 server-version + protobuf decode
  PR-4 datetime / message-type parse → Parse (lands new
       Error::parse_field / Error::parse_proto constructors)
  PR-5 cursor EOF + unexpected-response + decoder mismatch
  PR-6 new Error::ConnectionRejected(String) variant

Open items resolved with this audit:
  - Parent §5.3 Parse-shape: Option 4 — no-index constructors keep
    the variant tuple unchanged, encapsulate the fake-0 placeholder.
    Future promotion to Option<usize> remains possible.
  - PR-6 connection-rejected: new ConnectionRejected(String) variant
    (additive via #[non_exhaustive]), preserves io::Error detail and
    lets downstream code discriminate from generic ConnectionFailed.

Parent plan §5 item 1 → [~] with audit-count correction and link;
parent §5.3 → [~] with Option 4 resolution recorded.