[alloc+atomic] Fix missing cascade_discard_free_tail after non-tail dealloc/realloc

[williamwutq]

Description: cascade_discard_free_tail was only called from the explicit tail-discard path in dealloc. When coalescing in add_to_free_list produced a merged block ending at the stack tail, that block was never reclaimed — leaving the arena non-empty after all allocations were freed. Fixed by calling cascade after every add_to_free_list in both dealloc and realloc (Rust + C). This is purely a space reclamation bug — blocks are correctly marked free and coalesced, they just aren't discarded from the tail. Existing allocated blocks are never touched, and the free list remains structurally valid throughout. The only effect is the file grows larger than necessary and never shrinks back to the header size.

Important Feature: No
Type: Allocator - Correction; Allocator - Concurrent
Tests: Included
Feature Flags: alloc + set + atomic
Breaking change: No
New Types: None
Rust Only: No
Fuzz: Yes
Safety Review: Needed: Crash Safety, Invariants, Thread Safety

[Copilot]

Pull request overview

Fixes a space-reclamation bug in the first-fit allocator where coalescing during dealloc/realloc could create a free block at the arena tail that was never discarded, leaving the backing file larger than necessary after all allocations were freed.

Changes:

• Call cascade_discard_free_tail after every add_to_free_list in Rust dealloc/realloc to ensure tail free blocks are reclaimed.
• Mirror the same tail-cascade calls in the C allocator implementation.
• Document the behavior change (allocator comments + changelog entry).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/alloc/first_fit.rs	Adds tail-cascade calls after add_to_free_list and updates comments clarifying tail reclamation responsibility.
c/bstack_alloc.c	Adds tail-cascade calls after alff_add_to_free_list in dealloc/realloc.
CHANGELOG.md	Adds an Unreleased entry documenting the tail reclamation fix.

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