fix(libkernel): prevent POSIX timed-lock deadline overflow

[STUPIDCONTEM]

What

Avoid signed nanosecond overflow when converting absolute POSIX timed-lock deadlines. The shared condition/rwlock/semaphore helper now compares seconds and nanoseconds, computes their nonnegative difference, and clamps before converting to microseconds. The mutex timed-lock wrapper uses the same conversion instead of duplicating the overflowing chrono expression.

A valid distant deadline could previously become negative and cause an immediate ETIMEDOUT. This preserves the existing 32-bit microsecond wait cap, positive submicrosecond truncation, and argument-validation order; it does not extend that maximum wait interval.

FreeBSD stable/11 condition waiting carries the validated absolute timespec into the wait rather than narrowing the epoch to signed nanoseconds. The project represents both timespec fields as signed 64-bit values. Open #2064 and #2067 fix separate equeue/nanosleep conversions; searches found no competing implementation of this helper or the mutex correction.

Tested

Windows 10 Pro 19045, WinLibs GCC 15.2 UCRT posix SEH r7, Release, BUILD_TESTING=ON, main 6cc0503.

• A blocked rwlock with an INT64_MAX-second deadline fails against unchanged upstream with guest ETIMEDOUT (60), although its holder releases after 20 ms.
• Fixing only the shared helper exposes the independent mutex path: the matching blocked-mutex regression still returns 60. With both paths corrected, five focused runs pass. Workers are released and joined before reporting errors.
• Existing ordinary/past/invalid timeout tests remain; an added short subsecond deadline times out as expected. Parent reviewed unsigned subtraction, nanosecond borrowing, clamping and the real condition/rwlock/semaphore callers.
• With the separate build repair fix(build): declare SPIRV headers for Vulkan helper tests #2093, default and explicit libs builds passed and full CTest passed 494/494 in 34.62 seconds, with no skipped tests (combined head 018ad968). Conventions, whitespace and registration checks passed.

No Linux or title execution is claimed for this change.

Checklist

• Based on current main; no other open PR implements this correction
• One deadline-conversion topic
• No code comments added
• No silent stubs added
• No notes, reports or images added to the repository
• No third-party code added
• Generic behavior, not title-specific
• Depends on: fix(build): declare SPIRV headers for Vulkan helper tests #2093 for the full current-main graphics test build
• AI-assisted: yes; independently reviewed and tested