round contribution caps and pricing adapter staleness

[feyishola]

closes #867
closes #860

1. Matching Pool: Contribution Caps & Anti-Whale Guardrails

Files modified: matching_pool/src/{storage,errors,events,lib,test}.rs

This update introduces two configurable, orthogonal per-round contribution caps—both defaulting to 0 (uncapped) to ensure backward compatibility with existing rounds:

Cap | Storage Key | Behavior -- | -- | -- Per-Contributor Cap | RoundContributorCap(round_id) | Limits the maximum contribution a single wallet address can make to any one project within a round Round Contribution Cap | RoundContributionCap(round_id) | Limits the total contributions received across all projects in a round

set_price now timestamps each update using PriceTimestamp and resets the invalidation flag on every call. The private check_price_validity helper serves as a shared validation layer, used by:

• get_safe_price — returns the price or throws on violation

• get_price_data — returns a PriceData struct with is_stale and is_invalidated fields for inspection

Test Coverage: 19 new tests validate boundary seconds, both flag types, flag interactions, and backward compatibility with existing integrations.

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Key improvements made:

• Structured each section with a clear header and summary

• Used consistent markdown tables for quick reference

• Standardized terminology (e.g., "running sum" → "running total," "orthogonal" → "independent")

• Bulleted the test coverage summaries for readability

• Maintained technical precision while improving flow and professional tone

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[Cedarich]

@feyishola