WIP: preserve duplicate-key semantics in parse

[uvlad7]

What this changes

• JsonScanner.scan continues to return every matching occurrence. It cannot choose one duplicate-key value without changing its low-level matching semantics.
• JsonScanner.parse now follows JSON.parse object behavior: for repeated keys, the last source occurrence wins.
• Result assembly walks matches in reverse source order and inserts only a destination that has not already been populated. This preserves the last occurrence while avoiding parsing values that an overlapping matcher or an earlier duplicate would discard.
• Overlapping selectors, including a specific key selector together with ANY_KEY, therefore parse the same destination only once.
• Custom parsing is a block on JsonScanner.parse. The block receives the selected JSON fragment plus quirks_mode and symbolize_names; this permits callers to opt into parser-specific behavior such as allow_duplicate_key: true.

Documentation and tests

• Documents duplicate-key behavior, including the need to inspect all scan matches or use .last.
• Documents the custom parsing block with an allow_duplicate_key example.
• Covers duplicate keys, overlapping selectors, lazy single parsing, and custom parser behavior.

Verification

• Ruby 4.1: 59 examples, 0 failures.
• Ruby 2.3: 58 examples, 0 failures, 3 expected pending.
• RuboCop: clean.

This is deliberately a draft while the implementation and API are reviewed.