fix(encode/windows): resolve NVENC at runtime — AMD/Intel hosts no longer crash at start

The nvenc build linked nvEncodeAPI64.dll's entry points at load time, so a
--features nvenc binary hard-crashed on any box without the NVIDIA driver
(AMD/Intel). Entry points now come from a runtime LoadLibrary table
(encode/windows/nvenc.rs load_api); a missing DLL just falls through the
encoder auto-detect to AMF/QSV/software. The generated import lib and all its
plumbing (gen-nvenc-importlib.ps1, nvenc.def, PUNKTFUNK_NVENC_LIB_DIR,
setup-build-env wiring) are gone.

Live-validated on the RTX 4090 box (NVENC session, 7000+ frames).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/encode.rs

@@ -530,7 +530,7 @@ fn open_video_backend(
                 {
                     anyhow::bail!(
                         "NVENC requested/detected but this host was built without it — rebuild \
-                         with `--features nvenc` (needs the NVENC SDK's nvencodeapi.lib at link time)"
+                         with `--features nvenc`"
                     )
                 }
             }