Function setToothMobility

Set tooth toothNo's Miller mobility grade on the active chart from OUTSIDE the tooth-info panel. The panel's own #mobilitySelect writes mobility through applyToSelected(), which applies to the bulk selectedTeeth set — unusable here, since the full-mouth perio-chart overlay grid (P2 Task 2) edits one specific tooth at a time that is not necessarily activeTooth or selected on the base chart. This reuses the EXACT SAME mutation + render + notify steps that path takes (applyStateToSvg, updateToothTileNumber, re-syncing the panel when toothNo happens to be the active tooth, notifyStateChange) — no new mobility derivation or render logic, just a single-tooth entry point into it. Lazily initializes a never-touched tooth's state first, exactly like setPerioSite does. An unrecognized value (not in VALID_MOBILITY) is a silent no-op, exactly like every other per-tooth enum setter here.