Autocomplete --name with discovered speaker rooms

[JaviSoto]

This adds dynamic shell completion for the --name flag (Cobra RegisterFlagCompletionFunc) so room names are suggested based on discovery.

• Fixes bash completion for speaker names with spaces (e.g. Living Room) by escaping values.
• Adds a small disk cache (30s TTL) for completion results to reduce network discovery on repeated TAB.
• Adds unit tests by stubbing discovery (including names with spaces + cache behavior).
• Fixes a test race in internal/sonos/http_fallback_test.go (tests mutate a global function, so t.Parallel() caused cross-test interference).

Note: Codex with GPT-5.2 did the whole thing autonomously.

[steipete]

Thank you!