v4: Immutable config trees

[jtdub]

Description

Currently HConfigChild.text is mutable (modified during negate()), and comments/tags are mutable sets. This means remediation can mutate the source tree, requiring deep_copy() to avoid side effects.

Proposed Change

Make tree nodes immutable after construction:

• negate() returns a new node rather than mutating self.text
• comments and tags become frozenset (or copy-on-write)
• Tree modifications create new branches rather than mutating in place

Benefits

• Eliminates mutation bugs (remediation can't corrupt the source config)
• deep_copy() becomes unnecessary in most cases
• Thread-safe by default
• Easier to reason about

Breaking Change

Yes — any code that mutates child.text, child.comments, or child.tags directly.

Considerations

This is a high-effort change that touches nearly every method. May need a phased approach or a compatibility layer.