M2.14 IED Browser: distinguish FC CO controls from missing reads - #590

Operator-visible gap

In CSWI.Pos Data Model, SBO [CO], Oper [CO], Cancel [CO] were shown as Not read after explicit Read. These fields are command parameters, not scalar MMS read targets; blending them into incomplete-read labels makes successful reads appear unreliable.

Narrow fix

• A single IedBrowserReadPlan.UnobservedValue(FC) defines clear Control only state for FC CO and retains Not read for genuinely unread ST/MX/SG/CF leaves.
• Both lazily materialized DO/DA rows and canonical live-refresh path use it. No fake values or guessed authority.
• Tooltip explains that controlled operations belong to Signal Testing; Browser never operates or writes.
• Typed FC/structured-child regression verifies CO and ST remain isolated.

Acceptance boundary

Stacked on #589; exact-SHA Windows CI required. This improves truthful browser UX, not a claim that vendor SG enum unknowns are solved. No ARIEC pin, mainline, protocol or release changes.