feat(ktrace): added trace sessions and .ktrace extraction through /dev/ktrace

[FlareCoding]

Note

Medium Risk
New SMP-aware ring buffer and session state machine in the kernel plus devfs/syscall error plumbing; failures are mostly isolated but bugs could affect tracing correctness or concurrent open/start/stop behavior.

Overview
Adds kernel trace sessions with start() / stop() that clear per-CPU rings on start, drop events outside recording, stamp records with clock::now_ns(), and on stop close rings until reserved slots commit. Stopped sessions can be read as a versioned .ktrace blob (header, CPU table, oldest-first records, including wrapped rings) via /dev/ktrace/trace, with /dev/ktrace/control (start/stop writes) and /dev/ktrace/status (text snapshot). Boot registers these nodes through ktrace::register_devfs_nodes() after ktrace::init().

Introduces shared devfs::text_snapshot_node and append_str / append_u64 helpers (grow-on-read instead of fixed caps); sysstat drops its local snapshot node and uses this type for /dev/sysinfo/*. Plumbs fs::ERR_NODATA and EBUSY through resource providers and syscalls as ENODATA / EBUSY. Adds a userland ktrace app (start, stop, status, dump -o FILE) plus kernel tests for devfs snapshots and ktrace session/control/trace behavior.

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