refactor: reduce private Rust lint backlog

[limityan]

Summary

Completes the current non-blocking Clippy backlog pass for private Rust internals while preserving public API and product behavior.

• reduces the fresh three-surface warning inventory from 280 to 257
• resolves 23 selected diagnostics through private parameter grouping, zero-cost type aliases, private FFI naming cleanup, and supported dependency API usage
• preserves CLI, ACP, OpenCode, remote workspace, serialization, protocol, and public Rust interfaces
• adds focused regression coverage for event ordering, configuration values, result shaping, and Windows FFI layout
• does not add allow/expect exceptions and does not change CI enforcement

Deliberately retained backlog

Four private too_many_arguments diagnostics remain: three coordinator lifecycle helpers and one ACP session helper. Safe cleanup would require broader fixture or lifecycle restructuring, so they remain non-blocking rather than expanding this PR or weakening lint policy. Existing public-family and unsafe-related warning backlogs are unchanged.

Risk review

Two independent adversarial reviews covered architecture/concurrency/performance and product/API compatibility. Both concluded MERGE with no actionable findings. The review specifically checked ownership and evaluation order, async and cancellation sequencing, lock scope, allocation/clone behavior, FFI layout and ABI, error propagation, public compatibility, remote/OpenCode/ACP behavior, and test fidelity.

Verification

• fresh main/desktop/installer Clippy inventory: 257 deduplicated warnings, exactly 23 below baseline
• cargo clippy --workspace --all-targets
• cargo test -p bitfun-core --lib --quiet (981 passed, 1 ignored)
• cargo test -p bitfun-desktop --lib --quiet (94 passed)
• cargo test -p bitfun-agent-runtime --all-targets --quiet
• cargo test -p bitfun-agent-stream --all-targets --quiet (57 passed)
• cargo test -p bitfun-services-core --lib --quiet (73 passed)
• pnpm run check:repo-hygiene

The remaining warnings are pre-existing, explicitly outside this pass, and remain non-blocking.