chore(host/windows): clean up DDA capture — fix unused imports, quiet secure-desktop log, sane retry default

- Remove 4 unused imports (PCWSTR in composed_flip, anyhow macro + SizeInt32 in
  wgc, Write in wgc_relay).
- DuplicateOutput1 retry defaults to N=1 (immediate legacy): on the secure
  desktop DuplicateOutput1 is LOGON_UI-only so it always refuses, and the
  release-before-reduplicate + gentle recovery keep the legacy dup stable;
  retrying there only blocked. Still env-tunable (PUNKTFUNK_DUP_RETRY_N/_MS).
- Throttle the 'using legacy DuplicateOutput' warning (expected + once-per-gentle-
  recovery on secure) so a lock dwell doesn't flood the log.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/capture/wgc_relay.rs

@@ -15,7 +15,7 @@
 
 use crate::capture::dxgi::WinCaptureTarget;
 use anyhow::{bail, Context, Result};
-use std::io::{BufRead, BufReader, Read, Write};
+use std::io::{BufRead, BufReader, Read};
 use std::sync::mpsc::{Receiver, SyncSender};
 use std::sync::Mutex;
 use windows::core::PWSTR;