feat(html): link canonical topology endpoints

[nia-sg-bot]

Summary

• make canonical offline HTML relationship endpoints focusable links to their file/symbol evidence
• generate deterministic, safe fragment IDs from canonical object IDs
• cover anchor and relationship-link parity in the HTML formatter tests

Validation

• python3 -m pytest (175 passed)
• git diff --check

Part of #20

This is a contained accessibility and evidence-navigation slice; renderer extraction and the remaining legacy-path migration are still tracked in #20.

Summary by CodeRabbit

• New Features

  • HTML diagrams now provide stable in-page anchors for files and symbols.
  • Relationship endpoints link directly to matching objects for easier navigation.
  • Relationship endpoints without a matching anchor remain plain code text.

• Bug Fixes

  • HTML rendering now reports duplicate file or symbol IDs with a clear validation error, preventing ambiguous links.

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📥 Commits

Reviewing files that changed from the base of the PR and between 1482e0d and 205e690.

📒 Files selected for processing (2)

• diffgraph/formatters/html.py
• tests/test_html_formatter.py

🚧 Files skipped from review as they are similar to previous changes (2)

• tests/test_html_formatter.py
• diffgraph/formatters/html.py

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Walkthrough

The HTML formatter now validates unique file and symbol IDs, creates stable SHA-256 fragment anchors, assigns anchors to object articles, and links relationship endpoints when matching anchors exist. Tests cover links and duplicate-ID rejection.

Changes

HTML anchor links

Layer / File(s)	Summary
Generate and validate object anchors diffgraph/formatters/html.py, tests/test_html_formatter.py	render() rejects duplicate file or symbol IDs, builds SHA-256-based anchors, and passes them to rendering helpers. Object articles now include matching id attributes.
Link relationship endpoints diffgraph/formatters/html.py, tests/test_html_formatter.py	Relationship endpoints link to matching object anchors when available and otherwise render as plain code text. Tests verify anchors, links, and endpoint-specific aria-label values.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Feature

Merge Risk: ⚪ Minimal · up to 205e6

The HTML anchor and relationship-link behavior is internally consistent, with no actionable merge-blocking risk identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 30.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: adding links for canonical topology endpoints in the HTML formatter.
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.

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• Commit unit tests in branch nia/issue-20-html-topology-navigation

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Actionable comments posted: 1

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Inline comments:
In `@diffgraph/formatters/html.py`:
- Around line 58-71: Validate that all file and symbol IDs are unique before
constructing anchors or rendering objects in the HtmlFormatter flow. Reject
duplicate IDs early, before _object_anchor, _object_items, and
_relationship_items are invoked, while preserving normal rendering for unique
IDs.

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Run ID: 9b07789e-1c2b-4d5b-9cb2-33eebb626a98

📥 Commits

Reviewing files that changed from the base of the PR and between 4245d45 and 1482e0d.

📒 Files selected for processing (2)

• diffgraph/formatters/html.py
• tests/test_html_formatter.py

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