fix(mappings): carry OriginalValue and the snippet formatting through a rewrite

[ako]

Closes #379.

describe → exec — how a document is copied — silently dropped a mapping's
OriginalValue and reformatted the JSON structure's snippet. No build error
either way; pure diff churn against a Studio Pro original.

Found by round-tripping ako/TestApp's Mappings module, hand-authored in Studio
Pro rather than shipped in a marketplace module. All three documents are now
identical after describe → exec, and the executor reports Unchanged.

Neither default was right, and the split is what shows it

The old behaviour rested on mendixlabs#882, which measured two mappings a blank app
ships and concluded Studio Pro always leaves OriginalValue empty. At corpus
scale the opposite is more common — 2,322 of 3,042 value elements whose
structure carries a sample store it.

But "always copy" is not the fix either. The split is per document:

	mappings
carry the sample on every element	145
carry it on none	107
mixed	2

So which one a mapping gets is a property of how and when it was authored, not
something mxcli can derive. Always-copy is wrong for 107 mappings; always-empty
is wrong for 145.

A rewrite does not have to choose. It knows what was stored, so it carries it
forward — guard-don't-drop, ADR-0005 —
matching stored to rebuilt by JsonPath, because names and order can change
while the schema binding cannot.

mendixlabs#882's actual decision is left intact: a newly authored mapping still writes
empty. Its comment is corrected in place rather than deleted, since the decision
stands and only its measurement was too narrow.

The export side needed OriginalValue added to the semantic type and to both
readers — its codec writer hardcoded "" rather than carrying the field at all.

The snippet half

describe pretty-prints, so exec stored the pretty form and a snippet Studio
Pro had on one line came back multi-line. Same JSON, different bytes. The
rewrite now keeps the stored formatting when the JSON is semantically equal,
comparing decoded values rather than strings. Anything that does not parse
counts as different, so a malformed snippet is replaced rather than silently
kept.

Verification

• ako/TestApp: ItemImportMapping, Export_mapping and item_json all
  IDENTICAL after describe → exec; executor reports Unchanged for all
  three.
• Controls: reverting the carry reports Modified and loses the samples;
  reverting the JSON comparison fails the formatting and key-order cases.
• 6 new unit tests, including the CREATE IMPORT MAPPING does not persist the JSON member names it is given. mendixlabs/mxcli#882 control (a new mapping still writes
  empty) and the JsonPath-matching rule.

Full suite, go vet, gofmt and make check-mdl (462 PASS) are green.

A note for the next person

The general shape is worth keeping: when a measurement says "always X" from a
small sample and a wider one says "sometimes X", check whether the split is
per-document before picking a default. A per-document split usually means the
answer is "preserve", not "choose". Recorded in the finding.

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