fix(readme): add missing research directory to structure diagram

[ygd58]

docs/research/ (~540 lines: research/overview.md + research/gossip/{gossip,transport,decentralized-access}.md) was added in #8, but README's Directory Structure section was last touched in #5 - before #8 - and was never updated. Small, self-contained fix.

Summary by CodeRabbit

• Documentation
  • Updated the directory structure documentation to include the research directory for background research and references.

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[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between b76b387 and f8d184c.

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• README.md

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📝 Walkthrough

Walkthrough

The README directory structure now includes learn and research. The research entry identifies background research and references.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to f8d18

The README now documents the learn and research directories without affecting runtime behavior or public interfaces.

🚥 Pre-merge checks | ✅ 9

✅ Passed checks (9 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title follows the required format, uses an allowed type and domain, describes the README change, stays under 72 characters, and has no trailing punctuation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Scope Discipline	✅ Passed	The pull request changes only README.md. The diff updates the Directory Structure diagram by changing the learn branch from terminal to non-terminal and adding the research entry. This directly ma…
Behavior Safety	✅ Passed	PASS. The pull request changes only README.md. It updates a text-only directory diagram from how-to-guides/learn to include the existing research directory. No executable code, public entity, co…
Over-Engineering	✅ Passed	PASS. The pull request changes only README.md. The patch updates the directory tree from two entries to three entries and adds the existing research directory. It adds no cache, helper layer, public s…
Security	✅ Passed	PASS. The pull request changes only the README directory diagram. It adds the literal research directory entry and adjusts the tree branch marker. The changed lines contain no injection mechanism, i…

✨ Finishing Touches

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• Create PR with unit tests

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