Harden completion generation

[malh]

What changed

• add an interactive progress spinner for recursive help discovery
• allow global options before nested command paths in generated completions
• skip unsupported advertised subcommands while retaining strict resource-limit failures
• share the configured output budget across stdout and stderr
• sync generated files and their directory before reporting success
• report bulk-update read failures directly
• extend unit and live Zsh regression coverage

Why

Generated completions could remain pinned to the root context when flags preceded a subcommand, one unsupported advertised command could abort otherwise useful generation, and stdout/stderr each received the full configured output allowance. The write path also lacked crash-durability syncs.

Impact

Help-derived generation is more resilient and nested completion works for common flag-before-command invocations without weakening timeout or output protections.

Validation

• go test ./...
• go vet ./...
• tests/completionctl-integration.zsh
• tests/help-completion-live.zsh
• git diff --check