Resolve an MCOS object nested in a struct field or cell element

[ww-mw]

An MCOS object saved inside a struct field, a struct-array element or a cell element of a .mat file or a model workspace now becomes the node the same object gets at the top level. Until now it was never decoded:

Where	Before	After
strings_nested.mat mixStruct.p	<1x1 Simulink.Parameter>, no children	Parameter node, Value 7
strings_nested.mat mixStruct.s	<1x1 string>	"inStruct"
strings_nested.mat mixCell{2}	<1x1 string>	"inCell"

How

• Parser. Every class-17 element carries its own object handle as its fourth part. parseOpaque now keeps it: integer storage only, read in stored order (as MATLAB reads it), and bounded to its own part and its container. A malformed handle is recorded as refused, and the variable degrades exactly as before.
• Decoder. decodeMcosVariables resolves every opaque in a file, at any depth, against the one shared MCOS heap in a single pass, keyed by variable rather than by name. The existing range and class-match checks are unchanged. decodeMcosBlob keeps its by-name contract as a wrapper.
• Containers. MatNode and ModelNode attach the results in a side table (mcosDecodedTable.ts, a WeakMap), so the parsed variables themselves are untouched.
• Nodes. parseMatVariable's opaque arm, the one dispatch every nested value goes through, builds a resolved object with modelOpaqueMcosVariable, the factory top-level objects use, through NodeRegistry, so no module-load cycle is added.

Unchanged on purpose

• A cell's one-line summary and its Variable Editor grid still spell an object element as <1x1 Class>. Across 93 files, every container summary and cell grid is unchanged.
• A decoded object in a MAT struct field refuses a rename, like its sibling fields (fixesChildNames). This changes 11 nodes in .mat files and models and 0 in dictionaries.
• .sldd handling is untouched. -v7.3 stays refused.

Tests

• New fixtures written by MATLAB R2027a: test/fixtures/mcos/nested_objects.mat and nested_ws.slx, generated by make_nested_fixtures.m, with MATLAB's answers in two truth files.
  • They cover nested objects in struct fields at two depths, a struct array, cells and cells of cells, object arrays, and strings (scalar, array, empty, missing), plus a model workspace.
  • Each nested object is graded against a separately built top-level twin, recursively through every child row, and against MATLAB's recorded values.
• test/nestedMcos.test.ts covers the parser, the decoder, the hand-built malformed handles, and the handle read order against MATLAB.
• test/matStringOpaque.test.ts's nested-string pins are flipped deliberately.
• Every part of the change is pinned by a test that fails when that part is reverted (deliberate mutations, files restored byte-identically).

npm run verify is green: typecheck, build, smoke, 5,417 tests passed, 0 failed, check:pack, check:leak, check:browser.