feat: add AdaptiveLevelStrategy for runtime compression level tuning

[oferchen]

Summary

• Adds AdaptiveLevelStrategy trait + DefaultAdaptiveLevelStrategy in crates/compress/src/strategy/adaptive_level.rs for sender-side, per-block compression level adaptation.
• The default impl uses an EWMA of the observed compression ratio with configurable thresholds (poor_ratio_threshold ratchets level down to save CPU, good_ratio_threshold ratchets level up for better ratio) and clamps to a codec-appropriate LevelBounds (zlib 1..=9, zstd 1..=19).
• Wire-compatible: only the sender's encoder level changes; token framing, message headers, and codec advertisement are untouched. Never advertised as a protocol capability.
• Opt-in: the existing fixed-level encoder path stays the default. This trait is wired in only when an explicit configuration knob requests adaptive tuning.
• Re-exported from compress::strategy and follows the existing Strategy/Profile module patterns.

Test plan

• level_bounds_orders_arguments and level_bounds_clamps_within_range cover bounds construction.
• first_observation_seeds_ewma covers EWMA initialisation.
• monotonic_bad_ratio_decreases_level drives 50 incompressible blocks and asserts the level lands at bounds.min.
• monotonic_good_ratio_increases_level drives 50 highly compressible blocks and asserts the level lands at bounds.max.
• neutral_ratio_holds_level confirms ratios between thresholds keep the level steady.
• ewma_smooths_outlier confirms a single bad block after good ones moves the smoothed value but does not jump straight to it.
• reset_clears_history, rejects_non_finite_observation, for_zstd_uses_wider_bounds, alpha_is_clamped_to_unit_interval cover edge cases.
• cargo fmt --all clean.
• CI: fmt+clippy, nextest (stable), Windows, macOS, Linux musl must pass.