Test controlled README p95 threshold validation

[coenttb]

Closes #14.

Replaces the environment-dependent p95 wall-clock assertion with a controlled measurement whose median remains below the unchanged 100 ms threshold while its p95 is 101 ms. The test constructs the public .timed(threshold:metric:) trait and validates the same internal threshold result recorded by the trait.

Validation:

• fresh Workspace build
• fresh Workspace test: 130 tests passed
• swift-format 602.0.0: changed test clean; existing unrelated findings remain outside this PR
• SwiftLint 0.62.2: no new finding; the existing 29 trailing-comma findings are the exact frozen PR Clear the 0.3 SwiftLint trailing-comma baseline #13 scope