plain-forge rules review and fixing

[dusano]

I went through all the rules in plain-forge and fixed factual issues and added missing information.

Additionally, I've instructed Claude Code with the following prompt for each file in /rules:

Check all the facts in @forge/rules/X.md on real examples.

Claude Code generated examples that tested the rules and come with many suggestions how to improve the rules in plain-forge and are now part of this pull request.

Here is the list of changes and fixes:

• Adding 'module-structure.md'
• Clarifying difference between Liquid templates and import and requires module.
• Fixed a wrong fact in linked-resources.md.
• Making clear in 'bullet-continutation.md' that one statement per line is preferred convention and not a rule.
• Removing mentions of 'template' from required-concepts.md because it was introducing confusion.
• Adding to func-specs.md more details and rationale for the complexity limit.
• Providing more details in test-reqs.md on test types.
• More precise information in func-specs.md what 'requires' modules pass on.
• Removing a statement from impl-reqs.md that is not adding value while it might introduce confusion.
• A small factual fix for required-concepts.md.