feat(presenter): DRM card selection + a usable kmsdrm swapchain error

Both come straight out of the first compositor-less bring-up (P3), on a
two-GPU box:

PUNKTFUNK_DRM_CARD=<n> pins SDL's KMSDRM device index. SDL enumerates
/dev/dri/card* and takes the first it can open, which is regularly the
wrong one: it chose the card a live compositor already held DRM master on
and died at swapchain creation, while the idle card with the connected
display sat unused. Kept an explicit operator choice rather than
auto-detection — deciding "is this card already mastered" needs the very
ioctl that taking master IS, so any in-process guess would be fragile.

The swapchain error now carries what to actually check. "vkCreateSwapchainKHR:
Initialization of an object has failed" is useless to someone bringing up a
kiosk; on the kmsdrm backend it now names the pinned card and lists the
three real causes in order (no connected connector / another DRM master /
NVIDIA). Empty on every other backend, where it would be noise.

The NVIDIA note is measured, not guessed: on NVIDIA proprietary + kmsdrm,
Vulkan enumerates the GPU AND the display (VK_KHR_display reports the
connected HDMI connector) and still fails — as root, with
nvidia_drm.modeset=Y, on a card no compositor was using. Not permissions,
not DRM master; their direct-display path wants the display leased via
vkAcquireDrmDisplayEXT and SDL's kmsdrm surface path does not do that.

Plan: punktfunk-planning design/embedded-arm64-client.md §P3

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/pf-presenter/src/run.rs

@@ -360,6 +360,28 @@ fn run_inner(mut opts: SessionOpts, mut mode: ModeCtl) -> Result<Option<Outcome>
     // identity and the session window gets the default-Wayland icon (the Linux analog of
     // the AppUserModelID adoption above).
     sdl3::hint::set("SDL_APP_ID", "io.unom.Punktfunk");
+    // `PUNKTFUNK_DRM_CARD=<n>` → SDL's KMSDRM device index, for a compositor-less (kiosk/embedded)
+    // run. SDL enumerates /dev/dri/card* and takes the first one it can open, which on a
+    // multi-GPU box is regularly the WRONG one: measured on a two-card machine it chose the card
+    // a live compositor already held DRM master on and failed at swapchain creation, while the
+    // idle card with the connected display sat unused. There is no reliable way to pick from
+    // inside the process (detecting "already mastered" needs the ioctl that taking master IS), so
+    // this stays an explicit operator choice rather than fragile auto-detection.
+    // Ignored unless the kmsdrm backend is actually in use.
+    if let Ok(card) = std::env::var("PUNKTFUNK_DRM_CARD") {
+        if card.chars().all(|c| c.is_ascii_digit()) && !card.is_empty() {
+            tracing::info!(
+                card,
+                "PUNKTFUNK_DRM_CARD: pinning SDL's KMSDRM device index"
+            );
+            sdl3::hint::set("SDL_KMSDRM_DEVICE_INDEX", &card);
+        } else {
+            tracing::warn!(
+                card,
+                "PUNKTFUNK_DRM_CARD must be a card NUMBER (e.g. 0) — ignoring"
+            );
+        }
+    }
     let sdl = sdl3::init().context("SDL init")?;
     let video = sdl.video().context("SDL video")?;
     let events = sdl.event().context("SDL events")?;