fix(nvenc): stop telling operators to reboot when the driver version is fine

`NV_ENC_ERR_INVALID_VERSION` is how the driver reports two opposite failures, and we only
ever explained one of them. The message told the operator to update the NVIDIA driver or
reboot — correct for a genuine header/kernel-module skew, and actively misleading for the
field case behind it: a host that streams once per boot and then fails every later session
at the caps probe, until the *process* restarts. A version skew is static; it cannot come
and go inside one process, so that advice cost the reporter a reboot per stream.

Split the two on the only fact that tells them apart: whether a session has already opened
in this process. `nvenc_status` gains a `SESSION_OPENED` latch set right after every
successful `open_encode_session_ex` — both backends' caps probe and real open, plus the
Windows availability probe. No session yet, the version word really is in question and the
existing skew advice stands. A session already opened, and the kernel module demonstrably
accepted this build's version word, so the message now names per-process driver state and
points at the cheap fix (restart the host service, no reboot) plus a request for the log.

The load-time gate cannot serve as this discriminator: `NvEncodeAPIGetMaxSupportedVersion`
is a pure userspace query, so the classic "updated the driver, didn't reboot" skew sails
through it and only fails later at the open. Only a session that actually opened proves the
kernel module agreed.

The split lives in a pure `invalid_version(bool)` so both halves are unit-tested without
touching the process-wide latch. This is a diagnosis change only — it does not fix the
underlying field bug, whose root cause is still open.

Verified on Linux (192.168.1.25): clippy `-p pf-encode --features nvenc` clean, `cargo test
-p pf-encode --features nvenc --lib` 15 passed, rustfmt clean.

crates/pf-encode/src/enc/linux/nvenc_cuda.rs

@@ -870,6 +870,9 @@ impl NvencCudaEncoder {
                 e,
             ));
         }
+        // The handshake with the kernel module just succeeded — from here on, an
+        // `NV_ENC_ERR_INVALID_VERSION` in this process cannot be a driver version skew.
+        nvenc_status::note_session_opened();
         let wmax = self.get_cap(enc, nv::NV_ENC_CAPS::NV_ENC_CAPS_WIDTH_MAX);
         let hmax = self.get_cap(enc, nv::NV_ENC_CAPS::NV_ENC_CAPS_HEIGHT_MAX);
         let yuv444 = self.get_cap(enc, nv::NV_ENC_CAPS::NV_ENC_CAPS_SUPPORT_YUV444_ENCODE);
@@ -1067,6 +1070,7 @@ impl NvencCudaEncoder {
             }
             return Err(nvenc_status::call_err("open_encode_session_ex", e));
         }
+        nvenc_status::note_session_opened();
 
         let mut cfg = match self.build_config(enc, bitrate) {
             Ok(cfg) => cfg,