Support the .mdl model format, in both of its on-disk flavours

[ww-mw]

Reads a .mdl as readily as a .slx, and opens it to the same tree: the same
blocks, parameter usages, config sets, model references, and model workspace.
ingest, loadFromPath, loadDirectory and the model source adder all take
the new extension; .mdl gets the Simulink-model icon and class rather than
falling through to the MAT default.

A .mdl is two formats wearing one extension

Both are read, because both are files a user can still have.

• The modern flavour is an OPC text package — the identical part set a
  .slx zips, delimited by __MWOPC_PART_BEGIN__ lines instead of archived,
  with binary parts base64'd. Decoding that framing hands the parts to
  parseModelParts, which is parseSlx minus the unzip, so one reader serves
  both containers rather than two that have to be kept in agreement.
• The classic flavour is the pre-R2012 nested-brace text that a model which
  was never migrated still has. It shares no framing with anything: blocks are
  brace-nested property lists, and the model workspace is a uuencoded MatData
  record holding a 1xN struct array of Name/Value pairs rather than the
  struct-of-variables an .mxarray part holds. readMxArrayRecords splits the
  record framing out of parseMxArray so both interpretations share it.

parseModel sniffs which of the three it was handed and is the export to reach
for; parseMdl and parseSlx remain for a known format.

Keeping the two formats from drifting

The parameter-usage gate is now the shared isParamReference, called by both
parsers, so the classic flavour cannot disagree with the .slx about which rows
it surfaces. Three rules came out of holding it to that standard:

• BlockType, Name and SID are XML attributes in a .slx and ordinary
  properties in a .mdl, so they are skipped;
• CopyOfModelName is classic-only bookkeeping and is now in NON_PARAM_PROPS
  — leaving it in gave the classic file an extra Child / CopyOfModelName row
  the .slx of the same model did not have;
• BlockParameterDefaults holds Block nodes with no .slx counterpart, so
  the walk starts from the model's System children.

A model reference is named without an extension, so the entry takes the parent
model's own: a legacy hierarchy is legacy throughout, and labelling a .mdl
model's children .slx would link to nothing.

Verification

MATLAB stays the only source of truth. test/parity/matlab/gen_mdl.m writes a
second parity corpus — one modern .mdl, the R2011b and R2017b classic exports
of the same diagram, the .slx twin of each, an MCOS pair, and mdl_truth.json
recorded from the model rather than from any one file, so every flavour is
held to the same expectation. Written by 27.1.0.3353139 (R2027a Prerelease), the
same MATLAB as the rest of the corpus.

mdl.parity.test.ts (156 tests) checks each flavour against that truth and
against its twin, including a byte-level part comparison in which the modern
.mdl and the .slx differ only in the two per-save parts —
simulink/modelWorkspace.mxarray included, which is what proves the base64
decode rather than sampling a value out of it. mdlParser.test.ts (32 tests)
covers the framing and the grammar against synthetic bytes. drift.mjs now
regenerates both corpora.

Four divergences are real, and are asserted rather than assumed: a classic file
records no release and no UUID (both come back ''), spells lastModified as
MATLAB does rather than as ISO 8601, and has no archive, so rawContents and
zipEntries are null. A fifth is MATLAB's limit and not the format's —
exporting to R2011b drops the linked data dictionary, so the block row that
links into it is a plain string there and a link target in every other flavour.

Version

1.1.0. Additive on every published path; nothing removed or renamed. The one
signature change that could break a compile is ParsedSlx.rawContents and
.zipEntries becoming nullable, and ParsedSlx is not re-exported from
src/index.ts — the exports map publishes only . and ./node.

npm run verify is green: 3325 passed, 26 skipped, tarball dist-only (419
files / 292.9 kB). .mdl is read-only, like .slx, .mat and .prj; there is
no write-back path and none is claimed.