Record what MATLAB does when the default enumeral is deleted

[ww-mw]

The question

An EnumType has item1, item2, item3 and its DefaultValue is item2.
The user deletes item2. Should the displayed Value change?

Measured answer — yes, and the two flavours differ

Probed against MATLAB R2027a Prerelease 27.1.0.3393633.

	Design Data (EnumTypeDefinition)	Arch Data (archdata.EnumType)
removing the default enumeral	allowed; live object keeps the dangling name	allowed; default immediately reads the first survivor
cleared or re-pointed?	neither, genuinely still stored	cleared — remove the first too and the default follows
removing the LAST enumeral	allowed, down to zero	refused (CannotDeleteLastEnumeral)
assigning a bogus name	rejected (DynamicEnum_InvalidDefaultValue)	rejected, same identifier
on save	invalid key dropped from the file	same
on reload	'' → first enumeral	first enumeral

Both converge on the effective default becomes the first surviving enumeral, and
neither ever persists a DefaultValue naming an absent enumeral. Design data just
defers the tidy-up to save time.

Where we differ, and how bad it is

We keep the dangling name, so the Value column shows the deleted enumeral, no
enumeral gets the "current" icon, the dropdown displays a value outside its own
options, and we write that DefaultValue into the file.

Rather than assume the severity, the file our writer produces was handed to
MATLAB: it opens without error and silently repairs the default — both .sldd
formats, each in its own MATLAB session, because two dictionaries defining the same
enum in one session make MATLAB reuse the first definition and contaminate the
second result. So it is a display and round-trip fidelity defect, not corruption.

What is in this PR

Documentation and probes only — no behaviour change, no dist/ change. The fix is
identified in Simulink.EnumType.md and explicitly marked not implemented, so the
change to make it is a separate, reviewable commit.

Also on record: design data appends with appendEnumeral and removes by index;
arch data adds with addEnumeral and removes by name; neither add accepts a
bare name.