Suppress accessibility updates at rating boundaries

[garethpaul]

Summary

• Treat bounded accessibility increment and decrement actions as no-ops.
• Avoid duplicate completed-update callbacks and bounce feedback when the rating cannot change.
• Cover both minimum and maximum boundaries in native and portable tests.

Baseline

Accessibility adjustment at a rating boundary could still trigger update feedback and completion behavior even though the visible rating could not change.

Improvements

• Suppress boundary updates when increment or decrement would exceed the allowed rating range.
• Preserve successful accessibility changes within bounds.
• Add minimum/maximum regression and portable static contracts.

Validation

• The current exact head 4d13bc857c3fe32c12cb37e268d913ad2f70cb63 passed identical six-context portable matrices in two fresh clones.
• Three exact-head hosted checks succeeded, including the macOS baseline simulator/sample-build authority.
• No IP network operations occurred, and redacted history/current-tree secret scans found zero findings.
• Branch protection remains strict and no pull requests are open.

Risk

• Boundary gestures now intentionally produce no callback or bounce feedback when the rating cannot change.
• Local Linux validation cannot execute Xcode or simulator tests; hosted macOS remains the native authority.

Follow-Ups

• Keep minimum and maximum accessibility behavior covered by native XCTest and the portable baseline.
• Revalidate callback semantics if the rating range or accessibility interaction model changes.