feat(encode): add the InlineTables option

Assisted-by: DeepSeek V4.1 Flash

docs/API.md

@@ -429,9 +429,10 @@ the basic form, so the output always re-parses to the same value.
 
 ### Inline tables
 
-A table element of a value array is written as one `{a = 1, b = 2}` line while
-it fits. An inline table that would pass the hundredth column carries newlines
-and a trailing comma instead, which TOML 1.1 allows:
+A table element of a value array, and a sub-table inlined by
+[`InlineTables`](#compact-documents), is written as one `{a = 1, b = 2}` line
+while it fits. An inline table that would pass the hundredth column carries
+newlines and a trailing comma instead, which TOML 1.1 allows:
 
 ```toml
 arr = [1, {
@@ -444,6 +445,30 @@ The closing brace and the entries are indented one tab per nesting level, a
 nested table is measured on its own line, and the output re-parses to the same
 value either way.
 
+### Compact documents
+
+`InlineTables(threshold)` writes a sub-table as an inline table when its
+single-line rendering is at most `threshold` bytes, and as a table header
+section when it is longer. A document of small tables therefore grows shorter:
+
+```go
+out, err := interpres.NewEncoder().InlineTables(60).Marshal(cfg)
+```
+
+With `60` and a table of three short entries, the same value is written
+
+```toml
+server = {host = "127.0.0.1", port = 9090, tls = {on = false}}
+```
+
+instead of three lines under a `[server]` header and a `[server.tls]` section.
+A nested sub-table takes part in the same way, and the whole option is off at
+`0` or less. Two limits are deliberate. An array of tables keeps the `[[a]]`
+header form, because its inline form re-parses as a value array and would change
+the value's Go type. And because an inlined table is a value line, every one of
+them precedes the first header of its document, so a table inlined next to a
+header is not read back as part of that header's section.
+
 ### Cancellation
 
 `MarshalContext` and `(*Encoder).MarshalContext` accept a `context.Context`. The
@@ -536,12 +561,14 @@ encoder:
 | `GroupByKind(v bool)` | `true` | group entries as scalars, then sub-tables, then arrays of tables; `false` preserves declaration order |
 | `OmitEmptyArrays()` | off | skip `key = []` for empty scalar arrays |
 | `UseLiteralMultiline(threshold int)` | `0` | emit multi-line strings of at least `threshold` bytes as literal `'''...'''` |
+| `InlineTables(threshold int)` | `0` | write a sub-table inline when its single-line form is at most `threshold` bytes |
 
 ```go
 out, err := interpres.NewEncoder().
 	GroupByKind(false).
 	OmitEmptyArrays().
 	UseLiteralMultiline(80).
+	InlineTables(60).
 	MarshalContext(ctx, cfg)
 ```