orb: fix + re-enable the set_orb_visual voice tool (parked in #562)

[mabry1985]

Background

set_orb_visual (#560) lets the voice agent restyle the live orb (variant / palette / numeric params). It was disabled in #562 because it was buggy in practice. The handler is parked at agent/tools.py (the set_orb_visual_handler + the parked-tool note) and gated off via the settings toggle in agent/config_store.py.

Why it matters

• It's a shipped-but-dark feature.
• It blocks WebMCP: voice-edit the live in-app orb (demo) #534 (WebMCP: voice-edit the live in-app orb demo).

Work

1. Root-cause the bug (what made it buggy — bad param application? race with the orb store? invalid variant/palette handling?).
2. Fix + add coverage.
3. Re-enable the tool + the settings toggle default.

LOE: M (needs root-cause first). Priority: feature — unblocks the WebMCP demo (#534).