ifcviewer: move configureSurface into ViewportCore (#84-u)

The swapchain configuration path — WGPU_PRESENT_MODE handling,
mode-preference order (Mailbox / Immediate / FifoRelaxed / Fifo),
caps probe, cfg.alphaMode wiring, and the on-resize depth + MSAA +
HiZ texture reallocation + bind-group invalidation — all move to
ViewportCore. The only VW-side concern was a QString::fromLatin1 in
the advertised-modes log, which becomes a std::string concat with the
same output.

ViewportWindow's render() / resizeEvent / surface-outdated retry paths
call core_.configureSurface() now. createSurface() (which returns the
platform-specific WGPUSurface from the host) stays VW because Qt
platform discovery has to happen on the Qt side.

src/ifcviewer/ViewportCore.cpp

@@ -4450,3 +4450,135 @@ bool ViewportCore::raycast(const float origin[3], const float dir[3],
     out.world_normal[2] = best_normal[2];
     return true;
 }
+
+// ===========================================================================
+// Surface configuration (#84-u): configureSurface
+// ===========================================================================
+
+void ViewportCore::configureSurface(int width_px, int height_px) {
+    WGPUSurfaceConfiguration cfg = {};
+    cfg.device      = device_;
+    cfg.format      = surface_format_;
+    // CopySrc lets the screenshot path copy the surface texture back to
+    // host memory. Trivial cost on all known backends.
+    cfg.usage       = WGPUTextureUsage_RenderAttachment | WGPUTextureUsage_CopySrc;
+    cfg.width       = std::uint32_t(width_px);
+    cfg.height      = std::uint32_t(height_px);
+
+    // Present mode preference order: Mailbox → Immediate → FifoRelaxed
+    // → Fifo. Override with WGPU_PRESENT_MODE=...; otherwise pick the
+    // first mode the surface actually advertises (asking for one that
+    // isn't listed panics wgpu-native from Rust).
+    //
+    // Why Immediate sits above FifoRelaxed: Mailbox is the right answer
+    // for an interactive viewer (vsync-aligned, no tearing, 1-frame
+    // queue) but a meaningful subset of Linux Vulkan stacks (some
+    // compositors, some driver/WSI combinations) silently don't expose
+    // it. On those stacks Fifo's 2-3 frame queue doubles input-to-
+    // photon latency; Immediate can tear but keeps latency at one
+    // render-body. FifoRelaxed is the middle option.
+    WGPUPresentMode preferred[4] = {
+        WGPUPresentMode_Mailbox,
+        WGPUPresentMode_Immediate,
+        WGPUPresentMode_FifoRelaxed,
+        WGPUPresentMode_Fifo,
+    };
+    const char* pm_name = "mailbox";
+    if (const char* s = std::getenv("WGPU_PRESENT_MODE")) {
+        WGPUPresentMode override_pm = WGPUPresentMode_Fifo;
+        bool known = true;
+        if      (std::strcmp(s, "fifo") == 0)         { override_pm = WGPUPresentMode_Fifo;        pm_name = "fifo"; }
+        else if (std::strcmp(s, "fifo_relaxed") == 0) { override_pm = WGPUPresentMode_FifoRelaxed; pm_name = "fifo_relaxed"; }
+        else if (std::strcmp(s, "mailbox") == 0)      { override_pm = WGPUPresentMode_Mailbox;     pm_name = "mailbox"; }
+        else if (std::strcmp(s, "immediate") == 0)    { override_pm = WGPUPresentMode_Immediate;   pm_name = "immediate"; }
+        else {
+            known = false;
+            Log::warn()
+                << "[wgpu] unknown WGPU_PRESENT_MODE=" << s
+                << " (expected fifo|fifo_relaxed|mailbox|immediate);"
+                   " falling back to preference order";
+        }
+        if (known) {
+            preferred[0] = override_pm;
+            preferred[1] = WGPUPresentMode_Fifo;  // Fifo is the only guaranteed-supported mode
+            preferred[2] = preferred[3] = WGPUPresentMode_Fifo;
+        }
+    }
+
+    WGPUSurfaceCapabilities caps = {};
+    wgpuSurfaceGetCapabilities(surface_, adapter_, &caps);
+    auto supports = [&](WGPUPresentMode mode) {
+        for (std::size_t i = 0; i < caps.presentModeCount; ++i) {
+            if (caps.presentModes[i] == mode) return true;
+        }
+        return false;
+    };
+
+    // Diagnostic: dump the full advertised set on first configure. If
+    // Mailbox is missing here the driver doesn't expose it (drives the
+    // input-latency story); if Mailbox is listed but we still pick
+    // Fifo, the preference order has a bug.
+    if (!surface_configured_) {
+        std::string advertised;
+        for (std::size_t i = 0; i < caps.presentModeCount; ++i) {
+            const char* name = "?";
+            switch (caps.presentModes[i]) {
+                case WGPUPresentMode_Fifo:        name = "fifo";         break;
+                case WGPUPresentMode_FifoRelaxed: name = "fifo_relaxed"; break;
+                case WGPUPresentMode_Mailbox:     name = "mailbox";      break;
+                case WGPUPresentMode_Immediate:   name = "immediate";    break;
+                default: break;
+            }
+            if (i > 0) advertised += ", ";
+            advertised += name;
+        }
+        Log::info() << "[wgpu] surface advertises present modes: " << advertised;
+    }
+    WGPUPresentMode pm = WGPUPresentMode_Fifo;  // spec-guaranteed fallback
+    for (WGPUPresentMode candidate : preferred) {
+        if (supports(candidate)) { pm = candidate; break; }
+    }
+    switch (pm) {
+        case WGPUPresentMode_Mailbox:      pm_name = "mailbox";      break;
+        case WGPUPresentMode_FifoRelaxed:  pm_name = "fifo_relaxed"; break;
+        case WGPUPresentMode_Immediate:    pm_name = "immediate";    break;
+        case WGPUPresentMode_Fifo:         pm_name = "fifo";         break;
+        default: break;
+    }
+    wgpuSurfaceCapabilitiesFreeMembers(caps);
+    cfg.presentMode = pm;
+    if (!surface_configured_) {
+        const char* note = "";
+        switch (pm) {
+            case WGPUPresentMode_Mailbox:
+                note = " (vsync-aligned, no queue lag -- default)"; break;
+            case WGPUPresentMode_Fifo:
+                note = " (strict vsync, may queue 2-3 frames)";    break;
+            case WGPUPresentMode_FifoRelaxed:
+                note = " (adaptive vsync -- sync if in budget, tear if not)"; break;
+            case WGPUPresentMode_Immediate:
+                note = " (vsync OFF -- framerate uncapped, may tear)";        break;
+            default: break;
+        }
+        Log::info() << "[wgpu] present mode = " << pm_name << note;
+    }
+    cfg.alphaMode = WGPUCompositeAlphaMode_Auto;
+
+    wgpuSurfaceConfigure(surface_, &cfg);
+    configured_w_       = width_px;
+    configured_h_       = height_px;
+    surface_configured_ = true;
+    ensureDepthTexture(width_px, height_px);
+    ensureMsaaColorTexture(width_px, height_px);
+    ensureHizTextures(width_px, height_px);
+    // depth_view_ was just replaced; force the HiZ + edge bind groups
+    // to rebuild against the new view on next encode.
+    if (hiz_bind_group_) {
+        wgpuBindGroupRelease(hiz_bind_group_);
+        hiz_bind_group_ = nullptr;
+    }
+    if (edge_bind_group_) {
+        wgpuBindGroupRelease(edge_bind_group_);
+        edge_bind_group_ = nullptr;
+    }
+}