harden size calculations, bounds checks, and SHM layout validation

[metsw24-max]

adds overflow-safe helpers for size arithmetic and applies them to all allocation and layout calculations in both slotmem backends

rejects creation when item_size * item_num or derived sizes overflow preventing under-allocation that could lead to out-of-bounds access during slot indexing

replaces direct size expressions with validated basesize, inuse_size, and alloc_size to ensure all memory allocations are correctly bounded

adds checks in slotmem_get and slotmem_put to ensure dest_len and src_len do not exceed the fixed slot size, preventing out-of-bounds reads and writes via memcpy

reorders validation so that slot index bounds are checked before deriving pointers like inuse + id, avoiding invalid pointer computation

adds guards against overflow in pointer arithmetic size * id before computing slot addresses

validates shared memory layout during slotmem_attach by recomputing expected size from descriptor fields and ensuring it does not exceed the actual segment size

prevents use of corrupted or inconsistent shared memory segments that could otherwise lead to invalid memory access

[metsw24-max]

@notroj look into this

[notroj]

Thanks for the PR