Stabilize release UI validation and Xcode selection

[RNT56]

What changed

• prefer the GitHub runner image maintained stable /Applications/Xcode.app instead of the first lexicographic versioned install
• keep versioned stable Xcodes as fallbacks and prerelease installs last, with an explicit PINES_XCODE_PATH override
• give each hosted UI smoke shard one bounded retry on a newly cloned simulator while keeping the underlying Xcode invocation at one attempt
• expand only the outer Xcode job budgets to 120 minutes in CI and 150 minutes for release validation; the per-process watchdog stays at 12 minutes
• enforce the retry environment and outer budgets with workflow contract checks
• add regression tests for Xcode candidate ordering and UI shard retry/exhaustion
• document the seven-shard release policy and v0.1.1 preview procedure

Root cause

The initial release workflow ran the full UI target because it omitted the CI smoke-mode environment; #30 fixed that drift.

The first run of this PR then exposed two more infrastructure defects. Six smoke shards passed, while testArtifactsResearchComposerFlow hit Failed to get matching snapshots: Timed out while evaluating UI query on both fresh clones. The outer 90-minute job limit canceled the step just after the second failure, before the script could emit its final retry-exhausted error.

The job log also showed that select-xcode.sh chose Xcode 26.0.1. GitHub macOS images install several Xcode 26 versions and maintain the current stable release at /Applications/Xcode.app, but the old script accepted the first Xcode*.app glob entry. That silently pinned CI, CodeQL, and release validation to the oldest available toolchain.

No smoke journey was removed or weakened. A deterministic product failure must still fail independently on both clones, and each Xcode test process keeps its 720-second guard.

Validation

• bash -n scripts/ci/*.sh
• ShellCheck on all CI scripts
• actionlint on all workflows
• ruby -c scripts/ci/check-xcode-workflow-contracts.rb
• bash scripts/ci/test-select-xcode.sh
• bash scripts/ci/test-xcode-ui-shard-retry.sh
• bash scripts/ci/check-xcode-workflow-contracts.sh
• bash scripts/ci/check-public-hygiene.sh
• local stable selector probe: Xcode 26.6 with the iOS 26.5 SDK and Metal tools
• git diff --check