gateway: reconnect to the trading farm the auth server named (ibx#295)

[userFRM]

Problem

The auth server names the trading farm to use, and the reconnect path re-derived one instead of keeping it — so a session routed to a regional farm reconnected somewhere else.

What this changes

ReconnectAuth carries the trading host and farm as the auth response gave them, and the reconnect selects that route rather than re-deriving one.

The routing was unreachable

Nothing selected it. The reconnect credentials were left blank at construction for the caller to fill, and only the Python binding filled them:

host: String::new(), // Filled by caller (Python EClient or Rust API)

EClient::connect_inner never did. Every scheduler gates on a non-empty auth.host, so no Rust caller reconnected on any transport — farm, CCP or HMDS. The jittered backoff of #218 and the warm-up suppression of #219 were dead code for Rust callers.

The credentials are now a constructor parameter rather than a field set afterwards. into_hot_loop_with_farms takes a CallerAuth, both bindings pass one, and the setter that could be forgotten is gone — a binding that omits them does not compile. That is what closes the class rather than the instance; the compiler found two further construction sites in the bench harnesses while this was applied.

Breaking change to the low-level API

Gateway::into_hot_loop and into_hot_loop_with_farms take a CallerAuth. HotLoop::update_reconnect_auth is removed — pass the values at construction instead. Client/EClient and the Python surface are unchanged.

Closes #295. Closes #378.

Test plan

• The credentials being a constructor parameter is enforced by the compiler, not by a test: it found two further construction sites in the bench harnesses while this was applied.
• tests/farm_reconnect_poc updated to the new signature and checks clean.
• cargo check --offline clean on --lib, --lib --features python, --bins, --examples, and each integration target individually.
• tests/ib_paper_compat compared against a clean checkout of the base commit — identical sorted diagnostic sets.
• cargo test --offline --lib — only the two known config::expiry_tests failures, which fail on the base commit for missing legacy tzdata (fixed separately in config: resolve the legacy timezone names IB states its times in (ibx#335) #336).

[userFRM]

Closing this. It's in #409 along with the rest of the fork, which is easier to take in one piece than sixty separate branches.