ci: migrate validation matrix off retired macos-14 runner #577

Part of the coordinated macOS runner retirement train: basefoundry/base#2604

GitHub will retire the macos-14 hosted runner on November 2, 2026. The current origin/main validation matrix still selects macos-14.

Scope:

• Change the active hosted validation matrix to pinned macos-15.
• Keep the library's logical compatibility/release platform contract distinct from the GitHub runner label; do not rewrite historical or logical macos-14 release-BOM records merely because their hosted executor changes.
• Update current CI/support documentation to state that the macOS 14 compatibility path is exercised on the macOS 15 runner.
• Keep the Bash 4.2/system-Bash compatibility assertions intact.

Acceptance criteria:

• No active workflow selects macos-14.
• Current docs clearly distinguish macOS support identity from hosted runner image.
• Historical evidence and logical release-platform fixtures are unchanged unless a tested mapping requires an update.
• git diff --check and focused BATS validation pass.
• The PR links this issue and Base #2604.