Rename FormControlRange to OpaqueRange and incorporate TPAC design feedback

[stephanieyzhang]

This PR applies the changes requested during TPAC discussions (meeting minutes and summary) and recent feedback:

• Renamed the proposal and interface from FormControlRange to OpaqueRange and generalized it beyond form controls.
• Defined opaque strings in host terms (e.g., <textarea> / text-supporting <input> today; custom elements in the future).
• Updated all concepts, algorithms, and examples to use host-defined opaque strings instead of form-control-specific value strings.
• Clarified that authors obtain OpaqueRange only via host APIs (e.g., getValueRange()), not via construction or mutation.
• Removed all string-exposing behavior (e.g., toString()) and specified that startContainer / endContainer return null.
• Moved the old FormControlRange design into Alternatives considered section.

[anaskim]

LGTM, thanks!