fix(style-engine): resolve built-in Word heading styles when missing from styles.xml

[gpardhivvarma]

Summary

Closes #2805

• Add fallback to Word's built-in heading defaults (Heading1–Heading6) in resolveStyleChain() when a referenced style is not defined in the document's styles.xml
• Apply the same fallback in the basedOn chain walk so custom styles inheriting from built-in headings resolve correctly
• Add 9 unit tests covering fallback resolution, precedence, basedOn inheritance, and integration through resolveRunProperties and resolveParagraphProperties

Problem

Documents created with python-docx's add_heading() reference Word's built-in heading styles (Heading1–Heading9) without adding explicit <w:style> definitions to styles.xml. This is valid OOXML — Word has ~260 built-in styles that implicitly exist. SuperDoc's resolveStyleChain() returned {} for these, causing headings to render as unstyled body text.

Changes

File	Change
style-engine/src/ooxml/builtin-styles.ts	NEW — static BUILTIN_STYLE_DEFAULTS map with Heading1–6 properties matching Word defaults (font sizes, spacing, outline levels, theme fonts, colors)
style-engine/src/ooxml/index.ts	2-line fix: ?? fallback to built-in defaults in both initial lookup and basedOn chain walk
style-engine/src/ooxml/index.test.ts	9 new tests

How it works

// Before: missing styles return empty
const styleDef = params.translatedLinkedStyles?.styles?.[styleId];

// After: fall back to built-in defaults
const styleDef = params.translatedLinkedStyles?.styles?.[styleId] ?? BUILTIN_STYLE_DEFAULTS[styleId];

Document-defined styles always win (?? only activates when the left operand is undefined). Built-in headings set basedOn: 'Normal', so the existing chain walk picks up the document's Normal style for font family and body defaults.

Test plan

• bun test in style-engine — 137/137 passing
• 9 new tests: run properties, paragraph properties, precedence, basedOn inheritance, outline levels, basedOn chain with built-in parents, resolveRunProperties + resolveParagraphProperties integration
• Manual verification with python-docx add_heading() document

[chatgpt-codex-connector]

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[gpardhivvarma]

Addressed the Codex feedback — pushed a follow-up commit adding Heading7–Heading9.

Note: Title, Subtitle, ListParagraph, TOC1–TOC9 are also referenced across the codebase and could hit the same missing-style issue. Adding all of them would bloat this PR — the headings are the most impactful fix. Happy to add the others in a follow-up.