feat(encode): add the InlineTables option

Assisted-by: DeepSeek V4.1 Flash

interpres.go

@@ -234,8 +234,9 @@ type Unmarshaler interface {
 //     (offset date-time), and LocalDateTime/LocalDate/LocalTime (local
 //     variants). A date-time writes its seconds only when the value carries
 //     them, and drops the trailing zeros of a fractional second.
-//   - A table element of a value array is written as an inline table, across
-//     lines when it does not fit one.
+//   - A table element of a value array, and a sub-table inlined by
+//     Encoder.InlineTables, is written as an inline table, across lines when it
+//     does not fit one.
 //   - Values implementing Marshaler are encoded by calling MarshalTOML and
 //     using its result.
 //   - Values implementing encoding.TextMarshaler, and not one of the
@@ -272,6 +273,8 @@ func MarshalContext(ctx context.Context, v any) ([]byte, error) {
 //	                          a nil/empty []Item struct slice is still skipped)
 //	LiteralMultilineAt: 0     (always emit the escaped basic form, never a
 //	                          literal one)
+//	InlineTablesAt:     0     (always emit a table header, never an inline
+//	                          table)
 //
 // Use the chainable option methods to opt out. The option state is private;
 // callers that need the underlying knobs reach for the methods rather than
@@ -280,6 +283,7 @@ type Encoder struct {
 	groupByKind        bool // default true; set via (*Encoder).GroupByKind
 	omitEmptyArrays    bool // default false; set via (*Encoder).OmitEmptyArrays
 	literalMultilineAt int  // default 0; set via (*Encoder).UseLiteralMultiline
+	inlineTablesAt     int  // default 0; set via (*Encoder).InlineTables
 }
 
 // NewEncoder returns an Encoder with default options.
@@ -312,6 +316,24 @@ func (e *Encoder) UseLiteralMultiline(threshold int) *Encoder {
 	return e
 }
 
+// InlineTables sets the size limit, in bytes of the single-line rendering, at
+// which a sub-table is written as an inline table instead of a table header,
+// which makes a document of small tables shorter. Use 0 or any negative value
+// to disable (always emit a header).
+//
+// A sub-table is inlined only when doing so keeps every value's type: an array
+// of tables keeps its header form, because its inline form would re-parse as a
+// value array. An inlined table that does not fit the line is written across
+// lines, which TOML 1.1 allows.
+//
+// With GroupByKind(false) the layout is already for presentation only, and an
+// inlined table follows the same rule as any other value line: it lands in the
+// section of the header that precedes it.
+func (e *Encoder) InlineTables(threshold int) *Encoder {
+	e.inlineTablesAt = threshold
+	return e
+}
+
 // Marshal encodes v to TOML bytes. It is equivalent to calling Marshal with v.
 //
 // Marshal is equivalent to MarshalContext with context.Background.