perf: defer update notice version lookup

[jiminu]

Summary

• defer current-version lookup until a non-empty update cache is present
• preserve update notices, stale-cache cleanup, and background refresh scheduling
• cover cache-absent, cache-empty, newer, and stale cached-version paths

Verification

• bash tests/common_zsh_test.bash
• zsh -n config/shared/zsh/update-notice.zsh
• shfmt -d -i 2 -ci tests/common_zsh_test.bash
• git diff --check
• base benchmark, 30 iterations; cached interactive p50/p95 were lower in this local run (informational only)

Note

bash scripts/check.sh was attempted, but local test_editor_workflow_options_and_keymaps failed due an untrusted user Mise config (~/.config/mise/conf.d/selfishell.toml). The update-notice suite passed.