[RyeMutt]

Two follow-ups to the lock-type sweep (#591), one commit each.

Pool workers sleep until work or a deadline wakes them

On Windows every pool thread ran sleepy_robin, whose wait was Sleep(1) and whose wake did nothing. An idle worker woke every tick of the system timer to look for work, and picked work up, or came back from a fiber's timed wait, only on a tick. The viewer's window asks for 1 ms ticks, so each idle pool thread woke about a thousand times a second. Windows 11 gives a process whose window is hidden the full 15.6 ms tick, whatever it asked for.

sleepy_robin now waits on an auto-reset event, which a fiber made ready from another thread sets. Where a fiber's timed wait sets a deadline, it also waits on a high-resolution waitable timer, which the tick does not round. If either handle can't be created it uses round_robin's own wait and wake, so no path sleeps a tick.

Measured in threadpool_test at Windows' default tick:

	before	after
cycles an idle worker runs in 250 ms	758,090	0
median overrun of a 2 ms timed wait	13.4 ms	0.4 ms

Conflict ahead: #586 (rye/aldb) has its own event-based sleepy_robin, whose deadlines go through millisecond WaitForSingleObject timeouts that the tick rounds, and its own tests/threadpool_test.cpp. Whichever lands second merges the two; this scheduler is the one to keep.

Generated UUIDs are random v4, made on any thread without a lock

LLUUID::generate made a version 1 UUID (the time, a clock sequence and the network card's address) and handed out its MD5. The time and clock sequence lived in statics no lock guarded, so threads generating at once made the same UUID: eight threads making 25,000 each produced only about 140,000 distinct UUIDs out of 200,000, on every run. Its one lock was an LLMutex on a rare branch, which asserted if a coroutine reached it.

A UUID is now 122 random bits with RFC 9562's version 4 and variant, from the system's cryptographic generator (BCryptGenRandom, getrandom, arc4random_buf) through a buffer each thread keeps. uuid_time_t, cmpTime, getSystemTime, getCurrentTime and LLUUID::mMutex are gone. getNodeID stays for the machine ID and hashed unique ID, and bcrypt joins the Windows system libraries.

combine and the name-based generate(string) keep MD5. The server uses combine to make an upload's asset ID, and generate(string)'s callers name files and keys with it.

Testing

• Tests were watched failing first on the old code: the idle-cycles and timed-wait tests failed on the old scheduler, and the version/variant and cross-thread uniqueness tests failed on the old generator.
• Windows vs2026-os RelWithDebInfo: built on top of what Locks follow one rule: LLCoros where a coroutine reaches, LLMutex where only threads do #591 merged, with no warnings. The viewer links, and 336/336 tests pass across the llcommon, llcorehttp, alscript, llui, newview, llmessage and llinventory binaries.
• threadpool_test passed 10 runs in random order (0 idle cycles every run, median overrun 0.40–0.43 ms), and lluuid_test passed 5.
• develop has since merged Units are constexpr, convert by one compile-time map, and reject mixed dimensions by concept #592, Script Studio QA round ten: keys as uuids, linked prims, the link notice, event hovers #593 and Sky modernization 4: ALSky in place of the sky's objects and pools #584, which touch none of these files; the committed files are byte-identical to the tested ones.
• Not done: macOS and Linux builds (left to CI) and an in-world run.

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Reviewing files that changed from the base of the PR and between e9d2331 and 9ebf45f.

Files selected for processing: 7

• indra/cmake/Linking.cmake
• indra/llcommon/CMakeLists.txt
• indra/llcommon/lluuid.cpp
• indra/llcommon/lluuid.h
• indra/llcommon/tests/lluuid_test.cpp
• indra/llcommon/tests/threadpool_test.cpp
• indra/llcommon/threadpool.cpp

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