DESCRIBE of a real import/export mapping does not round-trip — 112 of 327 rebuild a different mapping, and the output parses #260

describe {import,export} mapping was run over all 327 mapping documents in the
eight demo apps (mx-test-projects/*.mpk) and each result fed back to
mxcli check:

The 112 are the dangerous ones: describe → edit → exec reports success and
quietly changes the model. The 132 include 17 mappings that use nothing outside
MDL's expressive range — those are pure describe/grammar defects, listed below.

1. An object element cannot address a nested member path

describe import mapping KrogerAPI.IM_ProductList (CitrusGrove) emits:

create KrogerAPI.Pagination_ProductRoot/KrogerAPI.Pagination = meta/pagination { … }

line 6:71 mismatched input '/' expecting {',', '}'}

importMappingChild's object form takes a single identifierOrKeyword after =,
not a jsonMemberPath (mdl/grammar/domains/MDLDomainModel.g4:471). #927 gave
value elements multi-segment paths; object elements never got them, and Studio
Pro produces them routinely — an object mapped several levels down with nothing
mapped in between.

2. An array/wrapper element prints as an empty value binding, and its subtree is dropped

printExportMappingElement (mdl/executor/cmd_export_mappings.go:134) branches on
elem.Kind == "Object". An element parsed with Kind == "Array" or "Wrapper"
falls through to the value branch. describe export mapping AgentCommons.EM_Agent:

    Versions = ,
    Variables = ,
    TestCases = ,

Empty right-hand side — and every child dropped: all of Version, Tool, MCP,
KnowledgeBase, TestCase. It happens not to parse, which is the only reason this
has not silently destroyed a mapping. The import printer has the same branch.

3. Three describe-only spellings the grammar has never accepted

For entity-less / association-less elements describe emits create . = x and
create ./Module.Entity = x (import), . as x and ./Module.Entity as x
(export). All four are parse errors — verified directly:

line 3:4  extraneous input '.' expecting the start of a statement
line 10:11 no viable alternative at input 'create.'

Either the grammar gains them or describe stops emitting them.

4. JSON member names that are not MDL identifiers

jsonMemberPath is identifierOrKeyword (SLASH identifierOrKeyword)* — no quoted
form, and / is the path separator. So these cannot be written or described:

• https%3A//sws.siemens.com/sam/claims/tenantId (TcConnector.IM_DecodedJWT)
• sws.samauth.ten, research%3Aread
• $type — 43 occurrences (OneHarness/CapitalConnector)

5 documents. describe emits them raw and the output fails at the %.

The silent-loss list

What the 112 drop, by cause: array root 79, message-definition source 70,
converter microflow 19, find+ignore 12, nested root 9, create+error 6,
XML attribute 3, XML schema source 3, allowOverride 2, custom handling 1,
mapping parameter 1. Each has its own issue; this one is for the four defects
above plus the round-trip guarantee.

Why CI did not catch it

make check-mdl runs mxcli check with no project, so it validates syntax and
nothing else. Every defect here needs a project or a decoded document to see. The
test that would catch all of them: for every mapping in a fixture project,
describe → re-execute into a copy → compare the decoded documents. The demo
.mpks are far too large for CI (2.5 GB), but a curated fixture carrying one
mapping per shape is small.

Measured on 70a12365. Full analysis:
docs/11-proposals/PROPOSAL_mapping_coverage.md §1, §5, §8.