ci: migrate macOS validation to macos-15 runner

[codeforester]

Closes #577.

Part of the coordinated train anchored by basefoundry/base#2604.

Changes:

• Moves the active macOS validation matrix to pinned macos-15.
• Keeps the logical macOS 14 compatibility/release-platform identity.
• Clarifies the runner/support distinction in current documentation.

Local validation:

• git diff --check passed.
• Full tests/validate.sh was exercised; two existing validate-driver shim tests fail locally before the migration-specific checks, so hosted validation is required for final acceptance.

[codeforester]

Review: LGTM, no blocking issues. Reviewed all 5 files at 5f5a7c4.

• The runner is selected in one place (the tests.yml matrix, consumed via runs-on: ${{ matrix.os }}); no other workflow or script names macos-14. The two remaining mentions (docs/release-process.md --platform macos-14 and tests/release-bom-row.bats) are the logical release-platform id, which the issue says to keep.
• The job name changes from Validate (macos-14) to Validate (macos-15), but the ruleset's required checks (base/issue-branch-policy, Product validation, Quality contract) do not name the matrix leg, so no required check is stranded.
• Hosted Validate (macos-15), Validate (ubuntu-24.04), the Bash 4.2.53 / 4.4.23 / 5.0.18 / 5.2.37 compatibility jobs, the release gates and the Beacon downstream smoke are all green, so the Bash 4.2 assertions are intact.
• Historical record handling is right: the v2.2.0 run note says explicitly that it used the macOS 14 image.

Non-blocking

1. The PR body says two existing validate-driver shim tests fail locally before the migration-specific checks. Hosted validation is green, so this looks environmental, but please say whether they fail the same way on main (or file an issue) so the local failure is not left ambiguous in the record.
2. One wording point, inline on support-matrix.md.

[codeforester]

A macOS 15 run does not exercise macOS 14-specific behaviour, so "macOS 14 compatibility exercised on the pinned … macOS 15 runner" slightly over-claims in an Evidence table. The issue asks for this phrasing, so it is fine to keep, but something like "macOS 14 is the supported floor; hosted validation runs on the pinned macOS 15 runner (no macOS 14 runner available)" is more precise. Same point as basefoundry/base#2605 (README L591-592), so worth settling once across the train.