fix(mempool3): increase SIZE_CLASSES to avoid buffer overruns on large objects

[shruti2522]

mempool3's size classes previously capped at 2048 bytes. When allocating an object larger than this (e.g. ICU structs), size_class_index_for would silently truncate the size to 2048 bytes in release mode. This led to try_alloc placing oversized objects into 2048 byte slots, causing severe buffer overruns and eventual malloc_consolidate heap corruption panics.

this patch expands SIZE_CLASSES up to 65536 and changes size_class_index_for to panic instead of truncating.

[Copilot]

Pull request overview

Expands mempool3’s slot size classes to support larger typed allocations (e.g., ICU-sized structs) and aims to prevent unsafe placement of oversized objects into too-small slots.

Changes:

• Extends SIZE_CLASSES from a 2048B cap up to 65536B.
• Adjusts (intended) oversize handling so allocations beyond the largest class should fail loudly rather than silently truncating.

Suppressed comments (1)

oscars/src/alloc/mempool3/mod.rs:35

• size_class_index_for still truncates oversize allocations in release builds: the bound check is a debug_assert! (compiled out) and the function returns idx.min(SIZE_CLASSES.len() - 1), which silently maps any size > last_class into the largest class. That contradicts the PR description and can still place too-large objects into an undersized slot (now 65536B).

const SIZE_CLASSES: &[usize] = &[16, 24, 32, 48, 64, 96, 128, 192, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536];

#[inline(always)]
fn size_class_index_for(size: usize) -> usize {
    // binary search over size classes

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