index: AnyTrigramIndex.candidates mmap_overlay ArrayList may leak on alloc failure

[justrach]

Problem

In the mmap_overlay branch of AnyTrigramIndex.candidates, a result ArrayList is created with ensureTotalCapacity. If toOwnedSlice fails (OOM), the ArrayList's backing buffer is not freed — there is no defer result.deinit(allocator) before the break :blk null fallback:

// src/index.zig — AnyTrigramIndex.candidates, mmap_overlay branch
var result: std.ArrayList([]const u8) = .{};
result.ensureTotalCapacity(allocator, merged.count()) catch break :blk null;
while (it.next()) |k| result.appendAssumeCapacity(k.*);
break :blk result.toOwnedSlice(allocator) catch null; // ← leaks result.items on OOM

The leak is proportional to merged.count() (can be large for common trigrams) and accumulates on every search under memory pressure.

Fix

Add defer result.deinit(allocator); before the ensureTotalCapacity line, and replace catch null with a proper errdefer or restructure to only call toOwnedSlice when allocation is guaranteed to succeed.

[justrach]

Fixed in release/v0.2.57 (commit 6ae58b1). The mmap_overlay candidates branch now properly calls result.deinit(allocator) in the toOwnedSlice catch path, preventing the ArrayList backing buffer leak.