Add performance regression coverage and contributor docs

[bryan-shea]

Summary

Add regression coverage for heavier workloads and document the main performance limits, tradeoffs, and contributor guidance.

Why

Once performance improves, it needs protection. Contributor docs also help prevent the same mistakes from wandering back in under a different haircut. fileciteturn7file2

Scope

In scope:

• add regression tests for large-file and large-workspace scenarios where practical
• capture representative heavier-use scenarios in the current test strategy
• document known hotspots, practical limitations, and reproduction guidance where helpful

Out of scope:

• full benchmarking infrastructure if it adds too much overhead now
• public guarantees that cannot be maintained

Done when

• core heavier-use scenarios have practical regression coverage
• contributors have useful performance context and reproduction notes