feat(timezone): runtime-registerable IANA aliases (env var + API)

[wboayue]

Summary

• Add register_timezone_alias(name, iana) and IBAPI_TIMEZONE_ALIASES env var so users can map gateway-supplied timezone strings without rebuilding the crate.
• User-registered aliases take precedence over the built-in TIMEZONE_ALIASES table — works for both new entries and overrides.
• Update the unsupported-timezone error message to point at both options so users discover the fix on the first hit.

Why

IB Gateway emits a free-form timezone string from the host OS locale (e.g. China Standard Time, Romance Standard Time, GB2312 mojibake). When the gateway sends a label not in our built-in table, the connection fails. Today the only fix is a new release with the alias added — slow for users and high-touch for maintainers.

Design

• static TIMEZONE_REGISTRY: LazyLock<Mutex<HashMap<String, String>>> seeded once from the env var (mirrors the pattern at src/stubs.rs:40).
• Lookup precedence: user registry → built-in TIMEZONE_ALIASES → mojibake heuristic → time_tz passthrough.
• Env-var format: name=iana;name=iana with whitespace trimmed; malformed entries are warned and skipped (env-var typos should not kill connection setup).
• Logs a debug! line on registry hit so users can verify with RUST_LOG=ibapi::common::timezone=debug that their alias is firing.

Out of scope (follow-up)

• src/market_data/historical/common/decoders/mod.rs — parse_time_zone panics on unknown names. Inherits the registry behavior, but the panic itself is a separate bug worth its own PR.

Test plan

• cargo +1.93.0 clippy --all-targets -- -D warnings
• cargo +1.93.0 clippy --all-targets --features sync -- -D warnings
• cargo +1.93.0 clippy --all-features
• cargo +1.93.0 test --features sync (180 passed)
• cargo +1.93.0 test --features async (105 passed)
• 10 new timezone unit tests cover registry precedence, env-var parsing (basic, malformed, whitespace, empty, empty-sides), and the public register_timezone_alias smoke path.
• test_parse_connection_time_unknown_timezone_errors extended to assert the error message mentions register_timezone_alias and IBAPI_TIMEZONE_ALIASES.
• Manual smoke against a live gateway with a synthetic env var.