feat: add proper indentation support for inline (nested) elements

[DecimalTurn]

No description provided.

[Copilot]

Pull request overview

This PR introduces style-aware indentation support when inserting into multiline inline arrays/tables (including nested inline elements) by adding an indentWidth formatting concept, auto-detecting it from existing TOML, and propagating it into writer insertion logic.

Changes:

• Add TomlFormat.indentWidth (default 2), validation, documentation, and auto-detection (including tab handling).
• Teach writer.insertInline / inline positioning to use an explicit indentation width when inferring first-row columns in multiline inline containers.
• Add a helper (prepareInsertedNestedInlineContainer) plus extensive new/updated tests covering indentation and nesting scenarios.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/writer.ts	Stores per-root indent width and uses it when inserting into inline containers to align new multiline rows correctly.
src/toml-format.ts	Adds DEFAULT_INDENT_WIDTH, indentWidth option, auto-detection logic, validation, and propagation through TomlFormat.
src/patch.ts	Propagates detected/explicit indent width into writer operations and prepares nested inline containers before insertions.
src/parse-js.ts	Propagates indent width into parseJS-generated CST roots and prepares nested inline insertions for arrays.
src/inline-layout.ts	New helper to align nested inline containers based on an existing template container’s multiline layout.
src/tests/toml-format.test.ts	Adds assertions for indent width defaults and detection (spaces and tabs).
src/tests/patch.test.ts	Enables an existing “multiline empty array” test and adds coverage for indent-width-sensitive behavior.
src/tests/patch.indentation.test.ts	New focused test suite for indentation preservation across many patch scenarios (root, tabs, nesting, edge cases).
src/tests/patch.fuzz.test.ts	Updates fuzz expectations to match the new indentation/layout behavior.
src/tests/patch.dotted-key-spacing.test.ts	Adds (currently failing) tests documenting desired dotted-key spacing preservation behavior.
README.md	Documents useTabsForIndentation and the new indentWidth option with examples.
docs/PLAN-Indentation.md	Adds a detailed design/plan document for style-aware indentation behavior and future work.

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