feat: add median aggregation for high-value oracle reads

[dangelo352]

Summary

• Adds a bounded recent PriceData sample buffer per asset in price_oracle.
• Wires get_price_high_value to require at least 3 fresh positive samples above the high-value threshold and return the median, while preserving get_price/get_price_valid latest-price behavior for low-level consumers.
• Uses the stricter of the dynamic high-value staleness window and OracleConfig.max_staleness_seconds for sample freshness.
• Covers too-few samples, stale samples, odd/even medians, outlier resistance, and checked overflow behavior.
• Updates oracle threat/admin docs for the new high-value trust assumptions.

Fixes #467.

Note: the issue text mentions get_price_for_high_value_operation, but Soroban contract function names are capped at 32 chars and that name is 34 chars. This PR wires the existing get_price_high_value entrypoint instead.

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-oracle-median-target cargo test -p price_oracle -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-oracle-median-target cargo test -p price_oracle high_value -- --nocapture