requesthandler: Measure GetStats CPU usage per session

[rdoupe]

Every client shared one os_cpu_usage_info_t (src/obs-websocket.cpp line 64). os_cpu_usage_info_query() moves its last times forward on every call, so a client's GetStats reported the CPU usage since the last GetStats from any client, not since its own. With two clients polling at different rates, the slower one saw only the slice since the faster one's last call: mostly 0, sometimes a whole core's worth. The query also ran without a lock, while requests run in parallel on the thread pool, which is a data race on the shared state. In the Rust obs-util it also means two &mut references to it at once.

Each session now has its own CPU usage state. It starts when the client connects and is queried under a lock, so cpuUsage covers the time since the same client's previous GetStats (or since it connected). Requests without a session (vendor requests) still use the shared state, now also under a lock. The cpuUsage description in the request docs and in docs/generated says so.

Measured end to end: OBS recording a 640x360 scene, client A polling GetStats every 50 ms and client B every 2 s. B's cpuUsage is compared with OBS's own CPU time over B's interval, read from /proc/<pid>/stat:

Build	Client B's cpuUsage (actual)
v0.1.0-beta.20261018 AppImage (submodule 0d663ee)	0.00% (5.37), 0.00% (4.81), 6.25% (4.65), 0.00% (4.71), 3.12% (4.59), 0.00% (4.50)
This branch, C libobs lane	4.38% (4.37), 4.34% (4.34), 4.38% (4.37), 4.34% (4.34), 4.35% (4.37), 4.34% (4.34)
This branch, Rust libobs lane	4.34% (4.34), 4.34% (4.34), 4.34% (4.34), 4.31% (4.31), 4.34% (4.34), 4.35% (4.37)

Tested in obs-rust/obs-studio with test_ws_cpu_usage, which sends GetStats through RequestHandler with two sessions. It burns CPU, polls from session a, idles as long, then polls from session b. At 0d663ee, b reports 0.000% against a's 6.250% in both libobs lanes. With this branch, b reports about half of a's usage (3.047% against 6.094%), 5 of 5 runs in each lane. A second case sends GetStats from 8 threads at once, with and without a session.

Refs obs-rust/obs-studio#1346. The obs-rust/obs-studio PR that moves the submodule to this commit carries the regression test and closes the issue.