chore: release 4.3.0

[hogan-yuan]

Summary

Bump version to 4.3.0 and promote [Unreleased] CHANGELOG to [4.3.0].

Changes in 4.3.0

Added

• All languages: FundamentalContext gains etf_asset_allocation(symbol) — queries GET /v1/quote/etf-asset-allocation for ETF asset allocation grouped by element type (Holdings / Regional / AssetClass / Industry)
• Rust: new public longbridge::counter module — symbol_to_counter_id, index_symbol_to_counter_id, counter_id_to_symbol, and is_etf
• Rust: QuoteContext gains symbol_to_counter_ids(symbols) and resolve_counter_ids(symbols)

Changed

• symbol_to_counter_id now also consults the embedded index and warrant directories

Fixed

• Refreshed the embedded US ETF list (4574 → 7250 entries) and added index (648) + warrant (17693) directories