Align the teaching source and language contract with compact defaults

docs/language.rst

@@ -171,11 +171,11 @@ These omissions are accepted in newly authored source 7, not only in schema-2
 compatibility input. Earlier schema admission of constructors and fields is
 still governed by its version rules.
 
-Canonical formatting writes the hydrated occurrence references and default
-``message_kind``, ``reply_to`` and ``values`` explicitly. An absent
-``operation`` remains omitted in canonical output. Lossless no-op updates and
-unrelated name edits retain authored omissions; canonical formatting is a
-different operation. An admitted incomplete occurrence can still produce model
+Canonical formatting omits optional occurrence references equal to ``none``,
+the default Asynchronous ``message_kind``, empty ``values``, and absent
+``operation``. Required fields of a selected constructor remain present.
+Lossless updates retain unchanged explicit defaults and authored omissions;
+canonical formatting is a different operation. An admitted incomplete occurrence can still produce model
 checks requesting its missing work, exchange, lifeline or referenced scenario.
 
 .. list-table:: Occurrence body constructors
@@ -933,18 +933,30 @@ DataTypeDefinition has exactly the following constructors:
 
    * - Constructor
      - Required fields
+     - Optional fields (omission value)
    * - Boolean
      - None
+     - None
    * - Integer
-     - ``minimum``, ``maximum``: optional signed 64-bit integers
+     - None
+     - ``minimum``, ``maximum``: signed 64-bit integers? (none)
    * - Real
-     - ``minimum``, ``maximum``: optional finite IEEE-754 binary64 values
+     - None
+     - ``minimum``, ``maximum``: finite IEEE-754 binary64 values? (none)
    * - String
-     - ``minimum_length``: u32; ``maximum_length``: optional u32
+     - ``minimum_length``: u32
+     - ``maximum_length``: u32? (none)
    * - Enumeration
      - ``literals``: unique nonempty, case-sensitive strings
+     - None
    * - Record
-     - ``supertype``: same-layer Record ref?
+     - None
+     - ``supertype``: same-layer Record ref? (none)
+
+Optional fields may be absent or explicitly ``none``. Selected String and
+Enumeration constructors still require ``minimum_length`` and ``literals``,
+respectively, including zero and an empty list. Selecting a constructor does
+not merge fields from the default String definition. Unknown fields reject.
 
 Bounds are inclusive and ordered; an absent bound means unbounded within the
 represented numeric domain. Integer comparisons MUST NOT convert to binary64.
@@ -1840,11 +1852,30 @@ Map order is irrelevant. The writer preserves signed coordinate zero and
 emits decimals that recover the original binary32 value.
 
 Canonical output uses LF, four-space indentation, explicit identities,
-defaults, and JSON-escaped strings. Elements are emitted in UUID order, then
+and JSON-escaped strings. It omits fields equal to the omission defaults of
+their actual declaration or selected constructor, as specified in the field
+tables. This includes an empty project description. It MUST NOT omit a required
+field merely because its value is zero, false, empty, or ``none``. Elements are emitted in UUID order, then
 relations; diagrams and their nodes follow, then artifacts, then traces.
 Fields use deterministic name order. Output ends with LF. Alias values come
 from the model, never from regenerating display names.
 
+Omission is context-sensitive: Scenario uses the source default MixedLegacy;
+BoundaryExchange and CommunicationMean have different orientation defaults;
+state declarations determine their state-kind discriminator. A nonempty or
+whitespace-only description remains exact. Present-empty optional strings or
+collections, unbounded multiplicity versus a finite default, selected operation
+intent, and negative coordinate zero remain distinct values. A nondefault
+geometry record retains all required coordinates or dimensions.
+
+Only specified optional fields inside selected constructors may be omitted.
+For example, Integer bounds and LegacyPortContract.interface may be absent;
+String.minimum_length, Signal.items, port-contract payload lists, and historical
+snapshot name/description fields remain required. A LegacyPortContract record
+still requires direction. Historical semantic baseline payloads are retained
+without generic empty/default pruning. Compact output MUST preserve every
+persistent model field after parsing, including these distinctions.
+
 Comments and input whitespace are lexical trivia to the canonical writer.
 Canonical export does not preserve them. The source-project editor and compiler
 preserve authored comments and whitespace on Apply while inserting missing
@@ -1890,11 +1921,19 @@ adjusts to the destination scope; other token spelling remains authored.
 
 Updates MUST NOT canonicalize surviving declarations in full. Header, field and
 nested value positions are derived from the existing syntax parser. Unchanged
-fields retain their exact bytes, including field order, omitted defaults, number
+fields retain their exact bytes, including field order, explicit and omitted defaults, number
 and string spelling, comments, indentation and line endings. Changed literals and
 references replace only the required tokens; a reference with comments between
-``@`` and its alias retains those comments. A previously omitted default is
-inserted only if its semantic value changes. New fields use the existing record's
+``@`` and its alias retains those comments. A previously omitted field is
+inserted only when needed to express the requested value in its new declaration
+context. This includes an unchanged value whose omission default changes with
+the declaration kind. For example, changing BoundaryExchange to CommunicationMean
+while retaining orientation true requires ``oriented true;`` if previously
+implicit. Unchanged explicit defaults MUST survive unrelated sibling edits,
+including persistence-serializer omissions such as ``operation none``,
+``port_anchors []`` and ``baseline.semantic none``. New fields and value fragments
+use compact generation while retaining required selected-constructor payloads.
+New fields use the existing record's
 indentation and line-ending style without reformatting its other fields.
 
 A constructor tag change retains shared nested fields in place. Removed fields,
@@ -2173,7 +2212,7 @@ Text edits describe a complete candidate and must remove or repair dependencies
 explicitly before Apply succeeds.
 
 8.1. Focused diagram removal
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Delete is scoped to the focused diagram and the exact visible selection.
 Text fields retain their own Delete behavior; diagram shortcuts MUST NOT apply
@@ -2294,8 +2333,9 @@ This compatibility applies equally to source and JSON. It adds explicit
 defaults without changing any prior identity, name, reference, order, geometry,
 artifact or trace field. The original V2 golden source/JSON in
 ``crates/oa-core/tests/fixtures`` MUST NOT be regenerated from the newer demo;
-they verify that every old persistent field survives. Canonical output writes
-the added defaults through explicit migration to the current schema 7.
+they verify that every old persistent field survives. Canonical output uses
+the current schema 7 and omits values equal to its specified source defaults;
+parsing recovers the complete migrated model.
 
 The V2 adapter preserves every original identity, alias, prose, parent,
 placement, trace and artifact. Generic ports become Legacy with their original