feat(memory): scale L1 audio cache cap to host memory profile (#331)

Add audio_cache_l1_max_bytes to MemoryProfile and wire it into
AppState's AudioCache constructor. Normal hosts keep the historical
400 MB cap; LowMemory hosts drop to 50 MB.

Without this, a Pi 3B (1 GB total) starts with the L1 cache cap set
at ~40 % of physical RAM. Even with the Feature #3 prefetch caps
already in place, the runtime watchdog from the previous commit
ends up firing more often than necessary because nothing prevents
a single L1 fill cycle from claiming 400 MB. Capping at 50 MB caps
how much the cache can grow before either: (a) eviction picks up
naturally, or (b) the watchdog steps in if the rest of the system
is also under pressure.

Pure refactor in lib.rs — replaces a hard-coded 400 * 1024 * 1024
with a profile field read. The default-constructed AudioCache
fallback (when disk-spillover init fails) keeps its own crate
default; no need to thread the profile there since the failure
path is rare.

Two tests added: a presence-bound assertion in the existing Pi 3B
case, and an explicit comparison that LowMemory's cap is strictly
smaller than Normal's.

cc

crates/qbz-core/src/system_capabilities.rs

@@ -43,6 +43,11 @@ pub struct MemoryProfile {
     /// (44.1 kHz / 16-bit FLAC) so each cached track stays under ~15 MB
     /// instead of ~60 MB.
     pub allow_hires_prefetch: bool,
+    /// Upper bound for the L1 (in-memory) audio cache. The default
+    /// 400 MB cap is sized for Normal-class desktops; on a Pi 3B (1 GB
+    /// total) that single subsystem could consume 40 % of RAM, which
+    /// guarantees swap thrash before the watchdog can react.
+    pub audio_cache_l1_max_bytes: usize,
 }
 
 impl MemoryProfile {
@@ -61,6 +66,7 @@ impl MemoryProfile {
                 max_initial_buffer_bytes: 2 * 1024 * 1024,
                 max_concurrent_prefetch: 2,
                 allow_hires_prefetch: true,
+                audio_cache_l1_max_bytes: 400 * 1024 * 1024,
             }
         } else {
             Self {
@@ -70,6 +76,7 @@ impl MemoryProfile {
                 max_initial_buffer_bytes: 256 * 1024,
                 max_concurrent_prefetch: 1,
                 allow_hires_prefetch: false,
+                audio_cache_l1_max_bytes: 50 * 1024 * 1024,
             }
         }
     }
@@ -238,6 +245,17 @@ SwapTotal:       2097152 kB
         assert_eq!(profile.max_concurrent_prefetch, 1);
         assert!(!profile.allow_hires_prefetch);
         assert!(profile.max_initial_buffer_bytes <= 256 * 1024);
+        // L1 cap must be a small fraction of total RAM — at most ~10 %
+        // of a Pi 3B, so we don't reserve four-tenths of memory for one
+        // subsystem on a 1 GB host.
+        assert!(profile.audio_cache_l1_max_bytes <= 100 * 1024 * 1024);
+    }
+
+    #[test]
+    fn audio_cache_cap_is_significantly_smaller_on_low_memory() {
+        let normal = MemoryProfile::from_total_kb(8 * 1024 * 1024);
+        let low = MemoryProfile::from_total_kb(938196);
+        assert!(low.audio_cache_l1_max_bytes < normal.audio_cache_l1_max_bytes);
     }
 
     #[test]