Centralize stdin payload and control-prefix semantics

[t-kalinowski]

Summary

Centralizes stdin payload preparation, control-prefix splitting, and timeout bundle reuse classification in src/stdin_payload.rs.

This removes duplicated stdin/control logic from worker request handling and response finalization, so Ctrl-C/Ctrl-D semantics and worker stdin newline handling have one source of truth.

User-facing changes

None intended. The repl tool interface is unchanged; this PR preserves and codifies stdin/control edge-case behavior.

Internal changes

• Added stdin_payload as the shared module for stdin payload construction, control-prefix splitting, and timeout bundle reuse classification.
• Updated backend driver payload preparation to delegate to the centralized helper.
• Updated server/response.rs to re-export and use the shared timeout reuse classifier.
• Kept R-specific newline canonicalization as a backend text-preparation step while sharing payload construction across backends.
• Added Zod protocol regressions for raw stdin byte preservation and Ctrl-D newline-tail handling.
• Updated snapshots where test helpers now send exact bare Ctrl-C/Ctrl-D control inputs.

Testing

• cargo +nightly fmt
• cargo check
• cargo build
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test