feat(market-data): harden bar continuity recovery - #159

Summary

• harden bar continuity gap recovery with bounded provider requests;
• validate that each successful gap response covers every expected timeframe slot after clipping to the actual request range;
• reject partial or out-of-range gap history without delivering it or advancing the delivery watermark;
• retain per-route continuity trust watermarks so terminal unusable history marks the earliest unresolved boundary;
• keep DEGRADED sticky across later unrelated successful gap repairs until the unresolved range is verified;
• report FAILED followed by DEGRADED when live delivery resumes without verified continuity;
• report the actual bounded history range in continuity telemetry;
• keep retry state value-owned, use owner-loop process() with capped exponential backoff, and use steady_clock deadlines;
• keep live buffering bounded and document the recovery contract in EN and RU guides.

This PR is intentionally stacked on feat/market-data-continuity / PR #158.

Verification

• market_data_continuity_test: 24/24, compiled and run directly with MinGW;
• market_data_continuity_example.cpp: standalone syntax check passed;
• git diff --check: passed.

The continuity suite covers missing-range start/end, clipping, actual bounded telemetry, sticky degradation after partial gaps and failed prefills, retry scheduling, buffer limits, stream mismatch, shutdown with in-flight history, and recovery failure semantics.