Replace Microsoft.Extensions.ObjectPool with built-in Roslyn-based pool

[jeffijoe]

Performance results

Single-threaded

Pool	Time
Roslyn-based	~9.1-9.2 μs
MS Pool	~13.4-13.5 μs
Improvement	~47% faster

Multi-threaded

Threads	Roslyn-based	MS Pool	Speedup
1	17.9 μs	23.4 μs	1.3x
2	67.7 μs	120.5 μs	1.8x
4	187.9 μs	418.3 μs	2.2x
8	452.9 μs	1,471.1 μs	3.2x

Downstream performance improvements (formatting)

• Single-threaded: 3-5% faster consistently
• 8 threads: ~1.7x faster

Trade-off

The Roslyn-based pool allocates more under heavy contention (181 KB vs 783 B at 8 threads) because the lock-free CAS approach falls back to the factory on failed compare-and-swap. But these are short-lived Gen0 allocations that are cheap to GC, a worthwhile trade for the throughput gains for regular workloads.