feat(engine): add pair value container and key-name affinity matching

[HarshK97]

Problem

1. When diffing key-value containers (like JSON objects, Go struct initializers, Python dicts, or Rust struct patterns), value subtrees inside matched pairs could stay unmapped if top-down passes missed them. So updating a value inside a key-value pair produced noisy Delete and Insert actions instead of an in-place Update.
2. Unmapped pairs with identical key names produced false Delete and Insert actions when surrounding container structures changed, because the engine lacked key-name affinity resolution for unmapped key-value pairs.
3. The language rules schema had no generic pairs field to declare key-value pair container types across grammars.

What Changed

• Added a pairs list field to Rules in internal/treesitter/rules.go and configured key-value pair containers across rules files for C, C++, Go, Java, JavaScript, JSON, Lua, PHP, Python, Ruby, Rust, TSX, TypeScript, and Zig.
• Added matchPairValues in internal/engine/matcher.go right after top-down and declaration matching. So when two key-value pairs are matched, their value children get connected automatically if their node types match and neither value node was mapped yet.
• Extended matchPairValues with key-name affinity matching so unmapped pairs fall back to matching by key label identity (getKeyLabel) and parent scope context (getParentPairKey).
• Centralized getKeyLabel in internal/engine/utils.go so key label extraction is shared across matchers.
• Added unit tests TestMatchPairValues and TestMatchPairKeyNameAffinity in internal/engine/matcher_test.go and updated integration baseline test data fixtures.

Visual Comparison

• Example : tests/testdata/json_schemastore_hugo_theme_config/

Before

After