Extract InheritanceRef value object from InheritanceLoader.get_source

[coordt]

Summary

• Extracts the ad hoc "N/name" string parsing/rewriting in InheritanceLoader.get_source into a small InheritanceRef value object (parse/format), per the coupling cleanup filed in InheritanceLoader/InheritanceMap: private numeric-prefix protocol split across two files #47.
• Rewritten extends tags now reuse the text actually captured from the {% extends %} tag rather than the loader's map key, preserving exact pre-refactor output even when the two differ (caught in code review of the first pass).

Test plan

• uv run pytest — 342 passed (pre-existing, unrelated test_git_commands.py errors only, reproduced identically on master)
• New unit tests for InheritanceRef.parse/format
• New regression test proving the rewrite uses the captured extends text, not the map key (verified it fails against the intermediate buggy commit and passes with the fix)
• ruff check/ruff format --check clean on changed files
• mypy clean on changed file (one pre-existing, unrelated SuperUndefined.__call__ note untouched by this diff)

Refs #47

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-09-27 14:12 UTC

[github-actions]

Version hint: patch
Current version: 0.9.4
New version (when merged): 0.9.5

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