fix(host): actionable NVENC error logging — drop misleading "(no NVIDIA GPU?)"

Every NVENC entry-point failure was annotated "(no NVIDIA GPU?)", which
misled triage: the direct-NVENC path only loads on a machine that HAS an
NVIDIA GPU. A Linux user hit NV_ENC_ERR_INVALID_VERSION at
open_encode_session_ex (past the NvEncodeAPIGetMaxSupportedVersion pre-flight
gate) — the signature of a userspace/kernel driver version skew that a host
reboot fixes — and the log pointed at a missing GPU instead. A restart did
fix it.

Add encode/nvenc_status.rs: a shared NVENCSTATUS -> cause mapper that folds
the real cause into the anyhow::Error at construction, so every downstream
{e:#} log (the encode-recovery loop, session teardown) improves for free.
INVALID_VERSION now reads "update the NVIDIA driver, or reboot if you just
updated it (a host restart is the usual fix)"; NO_ENCODE_DEVICE /
DEVICE_NOT_EXIST / INCOMPATIBLE_CLIENT_KEY (session-count limit) / OOM /
UNSUPPORTED_PARAM get their own glosses. The required API version comes from
the SDK consts so it stays correct across crate bumps.

Wire it into all NVENC entry-point failures in both backends
(encode/linux/nvenc_cuda.rs, encode/windows/nvenc.rs) — every open, init,
preset/resource/bitstream call.

Also: when the encode-recovery loop exhausts its in-place rebuilds it now
logs a clear terminal line with the underlying cause instead of the session
silently vanishing after the last identical "rebuilt in place" line.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/punktfunk1.rs

@@ -4919,6 +4919,16 @@ fn virtual_stream(ctx: SessionContext) -> Result<()> {
             if encoder_resets > MAX_ENCODER_RESETS
                 || !reset_stalled_encoder(&mut enc, &mut inflight)
             {
+                // Terminal: rebuilds are exhausted (or the backend can't rebuild in place). Say so
+                // plainly with the underlying cause — the per-reset lines above only ever repeat
+                // "rebuilt in place", so without this the session just vanishes. The error carries
+                // its own actionable text now (e.g. an NVENC version mismatch → "update/reboot the
+                // driver"), so this is the one line an operator needs.
+                tracing::error!(
+                    error = %format!("{e:#}"),
+                    resets = encoder_resets,
+                    "encoder did not recover after repeated in-place rebuilds — ending the video \
+                     session (see the error above for the cause)");
                 return Err(e).context("encoder submit");
             }
             tracing::error!(error = %format!("{e:#}"), reset = encoder_resets,