perf(daemon): coalesce live snapshots for 1.8.4

[ewrogers]

Summary

• coalesce recent field-map and message-dialog snapshot reads per client for up to 250 milliseconds
• force fresh captures for revision-sensitive field-map selection and message-dialog dismissal
• invalidate the retained snapshot when the event boundary is lost and preserve serialized worker backpressure
• document the behavior and prepare workspace metadata and the changelog for v1.8.4

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• mdbook build docs
• native Windows x64 rpc-client and rpc-daemon release build
• native Windows i686 loader and rpc-dll release build
• native Windows rpc-daemon tests and Clippy with warnings denied
• packaged da-rpc-v1.8.4-windows.zip and SHA-256 checksum with tools/package-release.ps1