Module arena

A bump arena: an explicit Allocator over one region.

{ Arena } :: import("std/arena");
arena := Arena.new(usize(1024) * usize(1024));
a := arena.allocator();
// … hand `a` to anything that takes an Allocator …
arena.deinit();   // PANICS if a block is still live

Placement, not lifetime: under reference counting every block still dies when its last reference goes away, and its release is routed back here by the owner prefix (std/allocator.yo). What the arena adds is where the bytes live — one contiguous region — and a checked end: deinit (and the arena handle's own dispose) panics while any block is live, so "the arena died under a live block" is a loud, defined failure instead of the use-after-free it is in Zig. A process-lifetime arena calls abandon() instead: tracking stops and the region is never released.

Freeing a block reclaims its bytes only when it is the most recent allocation; everything else is reclaimed by deinit. realloc grows the most recent block in place when the region has room.

Thread-safe: every operation takes the arena's spinlock, so blocks may be freed on any thread.

Under --debug-heap the exit report lists every arena still live (never deinit) or abandoned at exit, with its live blocks and bytes in use.

Stability

stable — new, allocator, scoped, deinit, abandon, live_blocks, used_bytes, capacity and is_released, frozen with the Allocator vtable (plans/archive/EXPLICIT_ALLOCATORS.md).