Merge feat/gpu-driven: HDR calibration and the HDR toggle crash fix

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

changes/hdr-calibration.md

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+bump: patch
+type: feat
+**HDR calibration, and two HDR fixes** — the HDR10 picture follows the player's display instead of one fixed curve.
+
+- **`r3d_hdr_calibrate(peak, paper, black)`** — nits as float bits: the brightest the display shows (100-10000),
+  where the picture's and the interface's white sit (80-1000, never above the peak) and how far the darkest shade is
+  lifted (0-5, fading out by paper white). The tonemap's and the overlay's HDR10 variants read them, and the display's
+  HDR metadata is sent again with the new peak. The defaults are the old constants: 1000, 200 and 0.
+- **`ov_hdr_nits(nits)` / `ov_hdr_paper()`** — what the overlay draws next is at that many nits on an HDR10 frame,
+  for a calibration screen's test patches; it closes the batch so far. No effect on an SDR frame.
+- **Fixed: a crash toggling HDR on the Vulkan renderer.** `gpu_caps_probe()` made and destroyed a second Vulkan
+  instance under NVIDIA Streamline's interposer; the next swapchain rebuild then called through a pointer that
+  instance had left behind, at address 0. With the Vulkan renderer running, the probe asks its instance instead.
+- **Fixed: yellow read as red in HDR.** The overlay drew sRGB values straight into the HDR10 swapchain (it now has an
+  HDR10 variant, chosen while `gpu_hdr_active()`), and the tonemap extended highlights per channel, which boosted a
+  bright yellow's red far more than its green; it now brightens the graded colour by one factor.