[Bug] Concrete `HeapBasedIndex` advertises non-indexed query scopes but implements them as empty traversal

[shaaravraghu]

Issue type
Correctness bug

Severity
High

Priority
P1

Affected modules
base-query

Affected files
base-query/src/main/java/build/base/query/HeapBasedIndex.java
base-query/src/main/java/build/base/query/Scope.java

Verification status
Source inspection

Problem summary
Scope.Direct, Scope.DepthFirst, and Scope.BreadthFirst are public query options, but the concrete HeapBasedIndex implementation returns Iterators.empty() for both traversal hooks. That means the public class does not actually traverse anything beyond indexed values.

Evidence
The scope enum exposes Direct, DepthFirst, and BreadthFirst.
HeapBasedIndex.traverse(Class<T>, Scope) returns Iterators.empty() at line 45.
HeapBasedIndex.traverse(Scope) returns Iterators.empty() at line 50.

Relevant references
base-query/src/main/java/build/base/query/HeapBasedIndex.java:43
base-query/src/main/java/build/base/query/HeapBasedIndex.java:49
base-query/src/main/java/build/base/query/Scope.java:33
base-query/src/main/java/build/base/query/Scope.java:39

Why this matters
The public API suggests additive traversal behavior for non-indexed scopes, but the default concrete implementation behaves as a no-op.
That makes scope selection misleading and can hide data that callers expect to be considered.

Reproduction steps

1. Inspect Scope.
2. Inspect HeapBasedIndex.
3. Observe that all traversal paths return empty iterators regardless of scope or type.

Expected result
Either the concrete HeapBasedIndex should implement direct and transitive traversal semantics, or the unsupported scopes should be rejected explicitly.

Actual result
The scopes are accepted but silently behave like "no traversal available."

Suggested fix direction
Implement real traversal for the concrete index, or fail fast for unsupported scopes so the public API cannot silently degrade.

Acceptance criteria
HeapBasedIndex either returns traversed objects for non-indexed scopes or throws a clear unsupported exception for those scopes.
Tests cover the concrete class rather than only custom subclasses of AbstractHeapBasedIndex.

Suggested labels
bug
query
api-behavior