refactor(validator_store): split validator_store/src/lib.rs into cohesive modules

[diegomrsantos]

Description

anchor/validator_store/src/lib.rs is currently very large and mixes multiple concerns.

Current snapshot (from unstable @ a27f6def2):

• ~2,980 lines
• ~78 function definitions
• responsibility mix: assignment caching/watch channels, QBFT/signature orchestration, proposer/attester/sync flows, slashing-protection lifecycle, and local helper types/tests

This makes the file hard to read, review, and safely modify.

Goal: split lib.rs into cohesive internal modules with no behavior change and no external API break.

Version

• Anchor branch/commit: unstable @ a27f6def2
• Rust: rustc 1.93.1 (01f6ddf75 2026-02-11)

Present Behaviour

Most validator-store logic lives in one monolithic file, making it difficult to:

• identify ownership boundaries
• reason about side-effects
• review refactors in small, safe increments

Expected Behaviour

Validator-store internals are organized into focused modules, for example:

• context/assignments cache and watch-channel flow
• proposer/block signing + builder-related flow
• attestation/aggregate flow
• sync committee flow
• slashing-protection helpers and shared utilities

AnchorValidatorStore remains the crate entry point and public behavior remains unchanged.

Steps to resolve

1. Create internal modules under anchor/validator_store/src/ and move logic by responsibility.
2. Keep public API stable (AnchorValidatorStore + existing trait behavior).
3. Move/adjust tests from lib.rs into per-module tests or mod tests files as appropriate.
4. Keep lib.rs as a thin composition layer (exports, wiring, constructor, module imports).
5. Validate no regressions:
   • cargo test -p anchor_validator_store
   • targeted integration tests that cover proposer/attester/sync signing paths

Acceptance criteria

• No functional behavior change
• No public API breakage for downstream crates
• lib.rs is significantly reduced and acts primarily as composition
• Existing tests pass; any moved tests continue to cover equivalent behavior