RUST-REF-005 - Decompose oversized Rust modules by responsibility

Summary

Reduce the Rust client's legacy oversized source files through small,
behavior-preserving extractions. Start with emulebb-core/src/lib.rs, which is
13,711 lines at the start of this item, then continue through the largest
responsibility-mixed production and test modules.

Release Triage (2026-09-30)

Post-beta. Responsibility-based decomposition remains worthwhile, but the
quality policy treats file size as an advisory and the candidate passes policy,
Clippy, tests, and the hosted OS matrix. No remaining module-size concern is a
release defect; resume this as small post-beta maintenance slices.

Invariants

• Preserve public Rust paths, REST/OpenAPI behavior, serialization, persistence,
  and configuration semantics.
• Preserve stock-compatible eD2K/Kad behavior and all VPN fail-closed boundaries.
• Keep module boundaries responsibility-based; do not split cohesive control flow
  solely to satisfy a mechanical line count.
• Remove private dead code and dependencies only when repository-wide references,
  all-target Clippy, and tests prove they are unused.
• Deliver each extraction as a separate reviewed and validated commit.

Work Plan

1. Split the inline emulebb-core tests into responsibility-focused modules.
2. Move the EmulebbCore facade and API implementations into focused modules.
3. Split download and Kad orchestration into explicit runtime responsibilities.
4. Use non-failing maintainability advisories to identify files that deserve a
   responsibility review; do not treat line count as a validation limit.
5. Continue with the largest mixed-responsibility modules in the core and
   protocol crates.

Acceptance Criteria

• emulebb-core/src/lib.rs is a thin crate root with stable public re-exports.
• Inline core tests are grouped by subsystem with unchanged assertions.
• Static IPv4 and P2P binding audits cover every relocated boundary.
• No repository-local Cargo target directory is created.
• Workspace format, Clippy, tests, Kad swarm, and VPN leak gates pass.
• Remaining large files are reviewed by responsibility without hard limits
  or per-file allowlist rationales.
• Substantial tests live in sibling test modules or integration-test trees;
  only focused white-box tests remain inline with production code.

Validation

• python tools\rust_quality_gate.py policy
• python -m emule_workspace validate --include-product-family --product-family-tier full
• Rust workspace, isolated Kad swarm, and VPN leak gates through supported
  workspace orchestration.

Current Evidence

• fcd6f36 makes the UI executable a three-line entry point over a library
  application boundary.
• 86b64da, 4a94087, 8caaef7, 5087dab, and 8a326e2 separate UI
  presentation, models, REST transport, background commands, and rendering from
  application wiring. Each extraction passed isolated all-target UI Clippy.
• 50070b3 moves the 1,796-line kad_store test suite into a sibling test
  module; all 41 focused tests pass and the production module is 539 lines.
• cf380fd replaces permanent lint allows with checked, reasoned expectations;
  two obsolete expectations and one ownership-only dead-code field suppression
  were removed during the review.