refactor(app_api): extract perf telemetry into own submodule

Lift PerformanceCpuSnapshot struct + the Linux /proc/stat parsers
(parse_proc_stat_line / read_total_jiffies / collect_proc_stats) +
the performance_cpu_snapshot Tauri command (~110 LOC) out of
app_api/core.rs into app_api/perf.rs. Self-contained CPU-usage
telemetry; nothing else in app_api references the helpers.

This is the "telemetry" slice cucadmuh's plan called out for splitting
core.rs's mixed runtime/platform/snapshot concerns. Other slices
(cli-bridge, runtime-control, platform window/scrolling) can follow.

app_api/core.rs: 235 → 122 LOC. lib.rs registration unchanged —
performance_cpu_snapshot is re-exported through `pub(crate) use
perf::*` in app_api/mod.rs.

src-tauri/src/lib_commands/app_api/mod.rs

@@ -2,12 +2,14 @@ use super::*;
 
 mod core;
 mod navidrome;
+mod perf;
 mod remote;
 mod integration;
 mod analysis;
 
 pub(crate) use core::*;
 pub(crate) use navidrome::*;
+pub(crate) use perf::*;
 pub(crate) use remote::*;
 pub(crate) use integration::*;
 pub(crate) use analysis::*;