refactor(store): move conflict publication domains into modules

[freezscholte]

Summary

• move conflict-set recording, stale-base failed-operation conflict data, merge-conflict recording, conflict preflight/resolve/list/show, and suggestion helpers into crates/forge-store/src/conflict.rs
• move publication trailers, exportable proposal metadata, publication row recording/checks, and publication lookup helpers into crates/forge-store/src/publication.rs
• preserve the existing store public API through lib.rs re-exports, including crate-private sync/review helpers
• leave clean merge success and integrity recomputation in lib.rs for later proposal/snapshot/evidence slices because those are not conflict-set ownership boundaries
• lower the store lib.rs line-count cap from 9,600 to 7,982 and update ADR-0001 with the NER-376 audit note

Verification

• rtk cargo check -p forge-store
• scripts/check-rust-line-count.sh
• rtk cargo test -p forge-cli --test forge_conflict_set
• rtk cargo test -p forge-cli --test forge_native_merge
• rtk cargo test -p forge-cli --test forge_accept_export
• rtk cargo test -p forge-cli --test forge_sync
• 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-376.
Related NER-371.

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