fix(fleet-waves): drop dead WavePreview / unfinished_waves

[NagyVikt]

Summary

Drops three rustc dead_code warnings that surfaced on every cargo build -p fleet-waves:

• struct WavePreview — never constructed in production code
• fn unfinished_waves — never called in production code; only reachable through the test below
• fn demo_plan (test helper) — orphaned once that test was removed

Also removes the matching test unfinished_waves_skip_completed_rows. Both the function and the test were exercised together but never wired into any rendering path — the live waves view uses wave_status / first_task_title directly, not WavePreview.

Test plan

• `cargo build --release -p fleet-waves` — clean, zero warnings (was 3)
• `cargo test --release -p fleet-waves` — 5/5 pass (lost the 1 test that depended on the dead helpers)

Net diff

`rust/fleet-waves/src/main.rs` -98 lines.