docs(tests): add test coverage plans

tests/_plans/models.md

@@ -0,0 +1,18 @@
+# Plan: pkg/models (20.4%)
+
+## Existing
+- `directives_test.go`
+
+## Gaps
+
+| File | Funcs | Cases |
+|------|-------|-------|
+| `models.go` | All `SQLName` methods, `UpdateDate`, `GetPrimaryKey`, `columnLess`, `GetForeignKeys` | Case handling, empty/nil maps, composite PK ordering, FK filtering |
+| `models.go` | `Init*` constructors (Database, Schema, Table, Column, Index, Relation, Relationship, Constraint, Script, View, Sequence, Domain, DomainTable, Enum) | Maps/slices non-nil, name set, defaults |
+| `sorting.go` | 20 Sort* funcs | By name and by sequence; ties; map variants return sorted slice; input not mutated where documented |
+| `flatview.go` | ToFlatColumns, ToFlatTables, ToFlatConstraints, ToFlatRelationships | Multi-schema, empty db, deterministic order |
+| `summaryview.go` | ToSummary | Counts across object types |
+| `directives.go` | `directiveFromAny` (22%) | Each input type branch, invalid type |
+
+## Data needed
+- One shared in-test builder for a multi-schema Database (reuse `tests/assets/dbml/complex.dbml` via reader only if no import cycle; otherwise build in code).