Fix TOML encoder to prefer readable table sections over inline tables

When converting from YAML/JSON to TOML, the encoder now always uses
readable TOML table section syntax ([section]) instead of compact inline
hash table syntax (key = { ... }), which better matches TOML's goal as
a human-focused configuration format.

Changes:
- decoder_toml.go: Mark inline TOML tables with FlowStyle so round-trips
  can be distinguished from YAML flow mappings
- encoder_toml.go:
  - encodeTopLevelEntry: use FlowStyle check instead of EncodeSeparate to
    decide inline vs table section (all block mappings now become tables)
  - encodeSeparateMapping: count FlowStyle children as attributes; use
    recursive encodeSeparateMapping for nested non-flow mappings
  - encodeMappingBodyWithPath: emit non-flow child mappings as sub-table
    sections instead of inline tables
- toml_test.go: add encode (YAML→TOML) test scenarios, update roundtrip
  expectations for inline tables (now expanded to table sections)

Agent-Logs-Url: https://github.com/mikefarah/yq/sessions/4824a219-6d5e-42e7-bca1-a8a277bf8c6a

Co-authored-by: mikefarah <1151925+mikefarah@users.noreply.github.com>

pkg/yqlib/doc/usage/toml.md

@@ -153,7 +153,9 @@ yq '.' sample.toml
 ```
 will output
 ```yaml
-name = { first = "Tom", last = "Preston-Werner" }
+[name]
+first = "Tom"
+last = "Preston-Werner"
 ```
 
 ## Roundtrip: table section
@@ -372,7 +374,10 @@ dob = 1979-05-27T07:32:00-08:00
 enabled = true
 ports = [8000, 8001, 8002]
 data = [["delta", "phi"], [3.14]]
-temp_targets = { cpu = 79.5, case = 72.0 }
+
+[database.temp_targets]
+cpu = 79.5
+case = 72.0
 
 # [servers] yq can't do this one yet
 [servers.alpha]
@@ -384,3 +389,20 @@ ip = "10.0.0.2"
 role = "backend"
 ```
 
+## Encode: Simple mapping produces table section
+Given a sample.yml file of:
+```yaml
+arg:
+  hello: foo
+
+```
+then
+```bash
+yq -o toml '.' sample.yml
+```
+will output
+```toml
+[arg]
+hello = "foo"
+```
+