The generated zsh completion script called compdef at source time,
which fails with 'command not found: compdef' when loaded before
compinit. Replace with a deferred registration that tries immediately,
and if compdef is not yet available, queues a self-removing precmd hook
that retries on first prompt.
Handles repeated sourcing (deduped hook entry) and shells that never
run compinit (completion simply never registers, matching zsh model).
Add real zsh integration test verifying no compdef error on source and
successful registration after compinit.
Fixes#14289
- Export `resolveCompletionCachePath` and `completionCacheExists` for external use
- Update `installCompletion` to require cache existence (never use slow dynamic pattern)
- Add `usesSlowDynamicCompletion` to detect old `source <(...)` patterns
- Add `getShellProfilePath` helper for consistent profile path resolution
- Update `formatCompletionSourceLine` to always use cached file
