Randomize cache index for singleton allocations

[anti-morma]

Fixes #1121.

What changed

• Randomize the cache index of normal allocations backed by singleton pages.
• Use at most 4032 bytes of existing size-class slack, in 64-byte increments, so the change does not reserve additional memory.
• Keep externally tracked allocations unchanged.
• Move debug padding bytes when the usable prefix grows.
• Add an API regression test for repeated allocations of the size reported in Slowdown when using mimalloc to allocate (many) buffers #1121.

Root cause

Large singleton allocations start on 64 KiB slice boundaries. A sequence of equally sized allocations can consequently map corresponding elements of every allocation to the same cache sets, causing severe conflict misses even though allocation itself is outside the timed region.

For the #1121 reproducer, all 1000 allocations were 64 KiB aligned before this change. With cache-index randomization, typically 10–24 of 1000 retain that alignment.

Results

Measured on an Intel Core i5-1235U, pinned to one CPU, using release builds from commit a3ca0e5e:

Allocator	Median runtime
glibc	0.703 s
mimalloc before	1.340 s
mimalloc after	0.269 s

The mimalloc virtual allocation span remains 345 MiB before and after. A separate loop of 100,000 allocation/free pairs at the affected size remained approximately 0.044 s both before and after.

Hardware performance counters were unavailable on the test host because perf_event_paranoid=4.

Validation

• API suite, debug build: 44/44 passed
• API suite, release build: 44/44 passed
• Multithreaded stress test: passed (4 5 2)
• git diff --check: passed

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