Add public accessor method for NodeId to extract inner usize value #112

Description:

Problem

The NodeId struct wraps a usize with pub(crate) visibility, making the inner value inaccessible from external crates:

#[derive(Debug, Hash, PartialEq, Eq, Clone, Copy, Ord, PartialOrd)]
pub struct NodeId(pub(crate) usize);

This makes it difficult for users of the library to:

• Serialize node IDs to external formats (JSON, databases, etc.)
• Use node IDs in external logging/monitoring systems that expect numeric identifiers
• Interface with other systems that require numeric IDs
• Debug and inspect node ID values programmatically

Currently, the only way to see the value is through Debug formatting ({:?}), which is fragile and not suitable for programmatic access.

Proposed Solution

Add a public accessor method to the NodeId struct:

impl NodeId {
    pub fn as_usize(&self) -> usize {
        self.0
    }
}

This follows common Rust patterns for newtype wrappers (similar to NonZeroUsize::get(), etc.) and maintains the type safety of NodeId while providing controlled access to the underlying value when needed.

Alternatives Considered

1. Make the field public (pub usize) - This would weaken the encapsulation and type safety that the newtype pattern provides.

2. Implement From<NodeId> for usize - This works but consumes the NodeId (though it's Copy, so not a real issue). The explicit method is more discoverable.