feat: add invalidate_cache() to R interface

[Copilot]

Exposes igraph_invalidate_cache() from the C library to R for debugging cache-related bugs. Without this function, determining whether user-reported bugs are cache-related from R was not possible.

Changes

• Added invalidate_cache() function in R/interface.R wrapping the existing invalidate_cache_impl()
• Added tests in tests/testthat/test-interface.R covering basic functionality and error cases
• Generated documentation with roxygen2, exported in NAMESPACE under "low-level operations" family
• Updated pkgdown configuration in _pkgdown.yml to add new "Low-level operations" subsection under Utilities

Usage

g <- make_ring(10)
is_simple(g)  # TRUE - cache populated

# For debugging: invalidate cache to verify results unchanged
invalidate_cache(g)
is_simple(g)  # TRUE - recalculated

# If results differ after invalidation, indicates a cache bug

The underlying C implementation and R bindings already existed; this PR provides user-facing access with proper documentation and categorization as a low-level utility function for debugging purposes.

• Fixes Add igraph_invalidate_cache() to R #1387

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add igraph_invalidate_cache() to R</issue_title>
<issue_description>This is necessary for debugging. I think this should be high priority. Without this, we won't be able to determine if a user-reported bug is cache-related from R. Such bugs should be very rare, but when they occur they are painful to debug.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add igraph_invalidate_cache() to R #1387

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[krlmlr]

@copilot: Agent instructions are updated, adapt. This is not a structural query, rather a low-level utility function. Needs a new family and adaptation to the pkgdown site index.

[Copilot]

@copilot: Agent instructions are updated, adapt. This is not a structural query, rather a low-level utility function. Needs a new family and adaptation to the pkgdown site index.

Changed from "structural queries" to "low-level operations" family and added new section in pkgdown configuration. (84ac7d1)