Split cultural-references.md into principles + source domains

[glacierphonk]

Problem

cultural-references.md is 191 lines covering two distinct things:

1. When/how cultural references work (the 4 conditions, failure modes, multi-level names) — principles needed during generation
2. Source domain catalog (Norse, Japanese, Arabic, Maritime, etc.) — reference catalog loaded only when exploring specific territories

The principles are compact (~80 lines) and always relevant. The source domain catalog is long and only relevant when the user chooses to explore cultural references.

Fix

Split into:

1. cultural-references.md (~80 lines) — keep the principles: when references work, when they fail, the "So What" test, practical checklist
2. source-domains.md (~110 lines) — move the source domain catalog (Norse, Japanese, Arabic, etc.) and the multi-level names section here

SKILL.md Step 3 would reference both, but Claude loads the source domains only if the user is exploring that territory.

Before submitting

Read CONTRIBUTING.md and run `markdownlint-cli2 '**/*.md'` before pushing.

Scope

Split one file into two + update references. Medium PR.