docs: fix two review findings from the reorg audit

An adversarial review of the reorg surfaced two doc inaccuracies:

- timers.md: a `time` code example still did `@import("runtime").time`, which
  no longer compiles — the runtime aggregator was deleted. Now `@import("time")`.
  (Reorg regression: the sweep rewrote `runtime.x` member access but not the
  `@import("runtime")` form inside a fenced code block.)

- sysv.md: `callconv(system.kernel_abi)` named a non-existent `system` module;
  `kernel_abi` lives in the boot-handoff contract (system/boot-handoff.zig),
  imported as `boot_handoff`. Now `boot_handoff.kernel_abi`. (Pre-existing bug —
  "system" was the file-path prefix mistaken for a module namespace — found and
  fixed opportunistically.)

docs/timers.md

@@ -68,7 +68,7 @@ Applications don't call the syscalls directly; they use `time`
 front door, not new mechanism.
 
 ```zig
-const time = @import("runtime").time;
+const time = @import("time");
 
 const start = time.now();                 // Instant — monotonic
 doWork();