Phase 6d: merge struct into property; storage follows use

`struct` and `property` had identical syntax and differed only in semantics, so
they are now one keyword: `property`. How a property is stored follows from how
it is used —

  - listed in a `model` or attached by `spawn`  -> an ECS component, kept in the
    engine's per-entity @S_/@H_ arrays and bound in queries (as before);
  - constructed with `new`                       -> a heap record with reference
    semantics (what `struct` used to be).

A program that declares only `property` records and functions — no `model`, no
`handler` — is not an ECS program: it gets record layouts and `new`, but no
entity storage, allocator, snapshot, or runtime splice. This is exactly the
shape of the Ludic compiler itself, whose Node/Tok/Buf/Val are now `property`.

Mechanics:
  - record layout (%Cmp_) now always emitted in the header (emit_head), so `new`
    works with or without the ECS; the per-entity arrays stay in
    emit_ecs_storage. %Str_ is gone — one layout prefix.
  - has_ecs() is now `has_systems() or has_models()`, not "any component"; a
    property alone no longer drags in the ECS runtime. Added has_models().
  - emit_new / member access / layout_ty / layout_node collapse onto find_comp.
    Dropped struct keyword, parse_struct, find_struct, is_struct_ty, N_STRUCT
    emission (the const stays at kind 0, the default node kind).

Migration done as two reseeds (the old compiler treats any component as ECS, so
it cannot see `property` records in the compiler source until has_ecs is fixed):
  A) teach the compiler property-as-record + fix has_ecs, keeping `struct`;
  B) migrate the compiler's own records to `property` and remove `struct`.

selfhost/tests/structs.ludic migrated (still prints 7 9 109 2 42). Vocabulary
drops `struct` from DECL (ludic_syntax.h, grammar, LudicTokens.kt). LANGUAGE.md
"Records" section rewritten. Reseeded (21613 lines); C-free fixpoint holds;
goldens identical; 17/17; vocab + doc-fences clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/editors/jetbrains/src/main/kotlin/io/ludic/ide/LudicTokens.kt

@@ -41,7 +41,7 @@ object LudicTokens {
  */
 object LudicVocabulary {
     val DECL = setOf(
-        "program", "import", "property", "struct", "model", "enum", "ui",
+        "program", "import", "property", "model", "enum", "ui",
         "const", "var", "fn", "extern", "handler", "entry"
     )
     val CLAUSE = setOf(

tools/editors/shared/ludic.tmLanguage.json

@@ -165,7 +165,7 @@
         { "name": "keyword.operator.logical.ludic", "match": "\\b(and|or|not)\\b" },
         { "name": "keyword.other.clause.ludic", "match": "\\b(phase|query|on)\\b" },
         { "name": "keyword.other.ludic", "match": "\\b(import|extern)\\b" },
-        { "name": "storage.type.ludic", "match": "\\b(program|property|struct|model|enum|ui|const|var|let|fn|handler|entry|state)\\b" },
+        { "name": "storage.type.ludic", "match": "\\b(program|property|model|enum|ui|const|var|let|fn|handler|entry|state)\\b" },
         { "name": "support.type.primitive.ludic", "match": "\\b(int|fixed|bool|entity|str|ptr|void)\\b" },
         { "name": "constant.language.boolean.ludic", "match": "\\b(true|false)\\b" },
         { "name": "constant.language.phase.ludic", "match": "\\b(Start|Input|FixedUpdate|Update|LateUpdate|Render)\\b" }

tools/editors/vscode/syntaxes/ludic.tmLanguage.json

@@ -165,7 +165,7 @@
         { "name": "keyword.operator.logical.ludic", "match": "\\b(and|or|not)\\b" },
         { "name": "keyword.other.clause.ludic", "match": "\\b(phase|query|on)\\b" },
         { "name": "keyword.other.ludic", "match": "\\b(import|extern)\\b" },
-        { "name": "storage.type.ludic", "match": "\\b(program|property|struct|model|enum|ui|const|var|let|fn|handler|entry|state)\\b" },
+        { "name": "storage.type.ludic", "match": "\\b(program|property|model|enum|ui|const|var|let|fn|handler|entry|state)\\b" },
         { "name": "support.type.primitive.ludic", "match": "\\b(int|fixed|bool|entity|str|ptr|void)\\b" },
         { "name": "constant.language.boolean.ludic", "match": "\\b(true|false)\\b" },
         { "name": "constant.language.phase.ludic", "match": "\\b(Start|Input|FixedUpdate|Update|LateUpdate|Render)\\b" }