refactor(store): move attempts snapshots domains into modules

[freezscholte]

Summary

• Move attempt lifecycle, workspace paths/markers, attempt resolution/list/show/attach, and materialization helpers into crates/forge-store/src/attempts.rs.
• Move snapshot save/restore/checkout/undo, expected content refs, latest snapshot helpers, and native history log/head walking into crates/forge-store/src/snapshots.rs.
• Preserve existing public API paths through lib.rs re-exports and keep cross-domain helpers crate-private.
• Lower the store lib.rs line-count cap from 5,962 to 4,624 and update ADR-0001 with the NER-378 audit note.

Verification

• rtk cargo check -p forge-store
• scripts/check-rust-line-count.sh
• rtk cargo test -p forge-cli --test forge_start_save
• rtk cargo test -p forge-cli --test forge_attempts
• rtk cargo test -p forge-cli --test forge_native_history
• rtk cargo test -p forge-cli --test forge_propose_check
• rtk cargo test -p forge-cli --test forge_review_surface
• rtk cargo test -p forge-cli --test forge_compare
• rtk cargo test -p forge-cli --test forge_conflict_set
• rtk cargo test -p forge-cli --test forge_native_merge
• rtk cargo fmt --all --check
• rtk cargo test --workspace — 603 passed across 61 suites
• rtk cargo clippy --workspace --all-targets -- -D warnings
• rtk bash scripts/ci.sh — PASS=95 FAIL=0

Fixes NER-378.
Related NER-371.

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