feat(core): read the Windows memory figures from GlobalMemoryStatusEx

detect_profile fell back to MemoryProfile::from_total_kb(u64::MAX) -- the top
tier -- on any machine without /proc/meminfo, and read_memory_pressure
returned None forever. The prefetch and buffer tiers were therefore pinned at
the top on Windows and the memory watchdog was blind: it continues on None, so
evict_l1_audio_cache and the prefetch halt could never fire at any pressure.

Both halves had to land together. With the old u64::MAX total, a working
pressure probe would have computed roughly 0 per cent available and reported
critical permanently.

ullAvailPhys is the analogue of MemAvailable: what an allocation can take
without going to disk. The Normal-fallback contract is preserved, so a probe
that fails still never throttles the machine. windows-sys 0.61 is already in
the workspace lock via qbz-local-catalog and qbzd, so nothing new is pulled.

Measured on a 32 GB box: 57.9 per cent available, class Normal, which is the
same class the old fallback produced -- the change is the true total and a
pressure figure that exists at all.

cc

crates/Cargo.lock

@@ -4179,6 +4179,7 @@ dependencies = [
  "serde_json",
  "thiserror 2.0.18",
  "tokio",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]

crates/qbz-app/src/memory_watchdog.rs

@@ -51,7 +51,8 @@ pub fn spawn(player: Arc<Player>) -> tokio::task::JoinHandle<()> {
         let mut interval = tokio::time::interval(Duration::from_secs(TICK_SECS));
         loop {
             interval.tick().await;
-            // None on platforms without /proc/meminfo — nothing to do.
+            // None where no memory probe exists (/proc/meminfo on unix,
+            // GlobalMemoryStatusEx on Windows) — nothing to do.
             let Some(pressure) = qbz_core::system_capabilities::read_memory_pressure() else {
                 continue;
             };

crates/qbz-core/Cargo.toml

@@ -30,6 +30,13 @@ anyhow = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 
+[target.'cfg(windows)'.dependencies]
+# W14: GlobalMemoryStatusEx stands in for /proc/meminfo. 0.61 is already in
+# the workspace lock (qbz-local-catalog, qbzd), so this pulls nothing new.
+windows-sys = { version = "0.61", features = [
+  "Win32_System_SystemInformation",
+] }
+
 [features]
 default = []
 # Tier 2 features (to be added later)

crates/qbz-core/src/system_capabilities.rs

@@ -141,14 +141,55 @@ pub fn pressure_from_figures(mem_available_kb: u64, mem_total_kb: u64) -> Memory
     }
 }
 
-/// Read /proc/meminfo and return a pressure snapshot relative to the
-/// detected MemoryProfile. Returns None on platforms without
-/// /proc/meminfo or when the file is unreadable.
+/// Windows' answer to `/proc/meminfo`: total and available physical memory,
+/// both in KiB, from `GlobalMemoryStatusEx`.
+///
+/// W14. Without this the profile falls back to the top tier and
+/// `read_memory_pressure` returns `None` forever, so the memory watchdog is
+/// blind on Windows and the prefetch/buffer tiers never step down on a small
+/// machine. `ullAvailPhys` is the closest analogue to `MemAvailable`: both
+/// mean "what a new allocation can expect without swapping".
+#[cfg(windows)]
+fn global_memory_status_kb() -> Option<(u64, u64)> {
+    use windows_sys::Win32::System::SystemInformation::{
+        GlobalMemoryStatusEx, MEMORYSTATUSEX,
+    };
+
+    let mut status = MEMORYSTATUSEX {
+        dwLength: std::mem::size_of::<MEMORYSTATUSEX>() as u32,
+        ..unsafe { std::mem::zeroed() }
+    };
+
+    // SAFETY: `status` is a live, correctly sized MEMORYSTATUSEX whose
+    // `dwLength` is set as the API requires; the call only writes into it.
+    let ok = unsafe { GlobalMemoryStatusEx(&mut status) };
+    if ok == 0 {
+        return None;
+    }
+
+    // The fields are bytes; the rest of this module speaks KiB.
+    Some((status.ullTotalPhys / 1024, status.ullAvailPhys / 1024))
+}
+
+/// Read the platform's memory figures and return a pressure snapshot relative
+/// to the detected MemoryProfile. Returns None on platforms with neither
+/// `/proc/meminfo` nor `GlobalMemoryStatusEx`, or when the source is
+/// unreadable.
 pub fn read_memory_pressure() -> Option<MemoryPressure> {
-    let content = std::fs::read_to_string("/proc/meminfo").ok()?;
-    let mem_available_kb = parse_meminfo_available_kb(&content)?;
-    let profile = memory_profile();
-    Some(pressure_from_figures(mem_available_kb, profile.mem_total_kb))
+    #[cfg(windows)]
+    {
+        let (_total_kb, avail_kb) = global_memory_status_kb()?;
+        let profile = memory_profile();
+        return Some(pressure_from_figures(avail_kb, profile.mem_total_kb));
+    }
+
+    #[cfg(not(windows))]
+    {
+        let content = std::fs::read_to_string("/proc/meminfo").ok()?;
+        let mem_available_kb = parse_meminfo_available_kb(&content)?;
+        let profile = memory_profile();
+        Some(pressure_from_figures(mem_available_kb, profile.mem_total_kb))
+    }
 }
 
 /// Pure detection given `/proc/meminfo` content. Falls back to Normal
@@ -164,6 +205,17 @@ pub fn detect_profile_from_meminfo(content: &str) -> MemoryProfile {
 /// profile on platforms without `/proc/meminfo` (macOS, Windows) or when
 /// the file is unreadable for any reason.
 fn detect_profile() -> MemoryProfile {
+    #[cfg(windows)]
+    {
+        // Same Normal-fallback contract as the meminfo path: a probe we could
+        // not run must never throttle the machine.
+        return match global_memory_status_kb() {
+            Some((total_kb, _avail_kb)) => MemoryProfile::from_total_kb(total_kb),
+            None => MemoryProfile::from_total_kb(u64::MAX),
+        };
+    }
+
+    #[cfg(not(windows))]
     match std::fs::read_to_string("/proc/meminfo") {
         Ok(content) => detect_profile_from_meminfo(&content),
         Err(_) => MemoryProfile::from_total_kb(u64::MAX),