Keep the string class when a Parameter Value is retyped

[ww-mw]

A Simulink.Parameter Value typed as a string literal came back as a char: enter "abc", the cell shows 'abc'.

ParameterNode.setProperty had arms for double, logical and string-array but none for a plain string, so the value fell to the catch-all that stores the parser's bare JS string — and a bare JS string is how this model spells a CHAR. A 1x1 string is ['abc'].

The read path already stored ['abc'] for the same value, so the two paths disagreed about one value. That is worse than a display glitch: the table seeds its in-place editor with the displayed text, so committing a string Value's own cell unchanged silently retyped string → char and dirtied the file.

Behind that sat a second gap on the binary save path. serializePropertyXml had no string arms either, so ['abc'] went down the numeric-array branch as Class="double" Dimension="1*1">abc (reparses as NaN) and an _array_type: 'String' wrapper hit the char fallback as [object Object]. Fixing only the model would have routed every string Value straight into it, so both are fixed here: strings now write MATLAB's envelope — an Element Class="string" wrapping a saveobj cell of char Elements, with Dimension omitted for 1x1. The spelling comes from the MATLAB-authored binary corpus under test/parity/artifacts, not from a guess. MatlabVariableNode's own string writer now delegates to that shared envelope so the format has one owner.

Tests

10 new tests, all of which fail on the pre-fix tree:

• parameterNode.test.ts — display, the cross-path invariant, commit-unchanged, embedded " escaping, and a control that a char stays a char
• dataNodeXml.test.ts — both string shapes, escaping, indentation, and a numeric control
• binaryWriteBackGate.test.ts — binary edit → rebuild → reopen
• parity/fidelity/parameter.fidelity.test.ts — a round trip parametrized over both formats

Left deliberately alone

• _serializeCellElementXml has the identical gap one level down, but its MATLAB spelling is not pinned by any corpus example and wants a probe first.
• A char literal still stays a char where MATLAB coerces to string — the divergence already recorded in test/parity/matlab/DESIGN.md, now pinned by a control test.