Perf: Use struct for WorkUnitResult

[ccastanedaucf]

Context

TargetEntry.ExecuteTarget() + TaskBuilder.ExecuteTask() allocate a large number of WorkUnitResult objects.

Nearly all of these allocations are a result of aggregating results across item buckets, as this produces a new WorkUnitResult instance on every single merge. This looks like an easy case for switching to a struct, given these are already treated as immutable and used as value containers (apart from 1 diff here).

Overall net ~40MB in this trace (after subtracting the small increase in types seen below + TargetResult + extra lock object).

Before

After

[Copilot]

Pull Request Overview

This PR converts WorkUnitResult from a reference type (class) to a value type (struct) to reduce memory allocations and improve performance. The change addresses excessive allocations occurring during target execution and task execution, particularly when aggregating results across item buckets.

Key changes:

• Convert WorkUnitResult class to readonly struct with immutable fields
• Update TargetResult to use a separate lock object instead of locking on WorkUnitResult
• Remove null checks and simplify code paths that no longer need to handle null references

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
WorkUnitResult.cs	Convert class to readonly struct, make fields readonly, update serialization logic
TargetResult.cs	Add separate lock object and setter for WorkUnitResult property to support struct assignment
TaskBuilder.cs	Remove unnecessary null check since structs cannot be null
TargetEntry.cs	Update assignment pattern for struct and remove null-conditional operator