TOML encoder: prefer readable table sections over inline tables

[Copilot]

When encoding to TOML from YAML/JSON, nested mappings were collapsed into compact inline table syntax (key = { ... }) instead of idiomatic TOML table sections ([key]). This affected both direct YAML→TOML conversion and round-trips through YAML.

Changes

Encoder (encoder_toml.go)

• encodeTopLevelEntry: Check TomlInline/FlowStyle — only nodes marked as inline stay inline; all block mappings become table sections
• encodeSeparateMapping: Count TomlInline/FlowStyle mapping children as hasAttrs (they render as key = { ... }); use recursive encodeSeparateMapping in the "no attrs" loop
• encodeMappingBodyWithPath: Emit non-inline child mappings as sub-table sections ([parent.child]) instead of inline tables

Decoder (decoder_toml.go)

• Set TomlInline: true on decoded TOML inline tables, distinguishing them from block mappings so they survive re-encoding as inline

Core (candidate_node.go)

• Added TomlInline bool field to CandidateNode — a TOML-specific marker invisible to the YAML encoder, copied by UpdateAttributesFrom so it survives DeeplyAssign deep-merges

Tests (toml_test.go)

• Add encode scenario type (YAML→TOML) and coverage for table section output
• TOML roundtrip tests preserved: inline tables stay inline, table sections stay as sections

Before / After

# input YAML
arg:
  hello: foo

# before
arg = { hello = "foo" }

# after
[arg]
hello = "foo"

YAML flow mappings (arg: {hello: foo}) and TOML inline tables are preserved as TOML inline tables, respecting the author's explicit style intent. TOML table sections and YAML block mappings both produce readable [section] headers.

[mikefarah]

@copilot the point of the roundtrip tests is to maintain the format as much as possible...this no longer does that.

[Copilot]

Fixed in d1ffec1. The root cause was that FlowStyle (used in the previous commit to mark inline tables) also affects the YAML encoder, which was changing the TOML→YAML decode output.

The fix introduces a new TomlInline bool field on CandidateNode — TOML-specific and invisible to the YAML encoder. The TOML decoder sets it for inline tables, UpdateAttributesFrom propagates it through DeeplyAssign merges, and the TOML encoder checks it to decide inline vs table-section output.

TOML roundtrip tests are restored to their original expected values: inline tables stay inline, table sections stay as table sections.