Refactor: remove assumption that the document root is an object

[dakusui]

Summary

Several parts of jq++ currently assume the processed document root is a JSON object
(map[string]any). That assumption is broader than issue #54 and should be removed
as an intermediate refactor before adding support for top-level YAML sequences.

Why this matters

Issue #54 surfaces this as a YAML problem, but the structural constraint is not
YAML-specific. The current pipeline expects an object root in multiple places:

• file loading and decoding
• inheritance resolution entry points
• key-side/value-side processing
• CLI processing/output path

As long as this assumption remains, support for top-level sequences is awkward to
add and likely to be incomplete.

Scope

Refactor the processing pipeline so the root document can be any JSON-compatible
value:

• object
• array
• string
• number
• boolean
• null

Object-only features such as $extends, $includes, and $local should remain
object-only and be applied only where semantically valid.

Expected outcome

After this refactor:

• the loader and main processing flow accept non-object roots
• traversal/evaluation logic works from arbitrary roots
• inheritance directives still behave correctly for object nodes
• follow-up work for issue jq++ cannot process top-level YAML sequences #54 becomes a narrower change focused on YAML top-level sequences

Related

• Feature: deep-merge into array elements by index #53
• jq++ cannot process top-level YAML sequences #54