fix(agent): vendor nvenc 并补 FORCEIDR 标志——修复运行期关键帧请求被静默忽略

根因（实测确认，非推测）
上游 nvenc 0.1.0 的 encode_picture 只设置 picture_type=NV_ENC_PIC_TYPE_IDR，
却从未设置 encode_pic_flags=NV_ENC_PIC_FLAG_FORCEIDR。NVENC 会据此忽略请求、
继续输出 P 帧。首帧之所以「看起来正常」，是因为 frame_idx==0 本来就强制 IDR，
与该字段无关——这也是该缺陷长期未被发现的原因。

证据（同一 frame_count=4，修复前后）
- 修复前：输出 NAL 首字节 0x61 → 0x61 & 0x1F = 1 = 普通 P 片
- 修复后：输出 NAL 首字节 0x65 → 0x65 & 0x1F = 5 = IDR 片

影响
这解释了构建运行手册 §6.1 记录的「静止桌面黑屏」：NVENC 路径的重编 IDR 从未
真正生效，KeyFrameReq 拿不到 IDR 就无法从解码错误中恢复。

改动
- 新增 vendor/nvenc（上游为 MIT 许可，可自由修改分发，符合商用要求）
- vendor/nvenc/src/safe/encoder.rs：picture_type=IDR 时置 ForceIDR；
  IntraRefresh 时置 ForceIntra。改动处有「本项目补丁」注释标记
- agent/Cargo.toml：nvenc 改为 path 依赖指向 vendor 版本

建议后续向上游提 PR（github.com/AlsoSylv/nvenc），以降低长期维护成本。

验证：跳帧后发 KeyFrameReq，第 3 帧即收到 IDR 并解码成功

vendor/nvenc/Cargo.toml

@@ -0,0 +1,63 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2024"
+rust-version = "1.85.1"
+name = "nvenc"
+version = "0.1.0"
+authors = ["AlsoSylv"]
+build = false
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Wrapper for the nvidia NVENC API"
+readme = "README.md"
+keywords = [
+    "nvidia",
+    "graphics",
+    "media",
+    "video",
+    "ffi",
+]
+categories = [
+    "multimedia",
+    "external-ffi-bindings",
+]
+license = "MIT"
+repository = "https://github.com/AlsoSylv/nvenc"
+
+[lib]
+name = "nvenc"
+path = "src/lib.rs"
+# 上游 README 的示例是已过时的 Linux GLX 用法（Session<NeedsConfig> 等类型已不存在），
+# 直接跑 doctest 会失败。本项目只做 vendor 补丁、不维护文档示例，故关闭。
+doctest = false
+
+[[example]]
+name = "simple_encode"
+path = "examples/simple_encode.rs"
+
+[dependencies.bitfields]
+version = "1.0.2"
+
+[dependencies.libloading]
+version = "0.8.9"
+
+[target."cfg(windows)".dependencies.windows]
+version = "0.62"
+features = [
+    "Win32_Graphics_Dxgi_Common",
+    "Win32_Graphics_Direct3D",
+    "Win32_Graphics_Direct3D11",
+]