Grid-Based Match-3 Chain Reaction Logic

Act as a Game Logic Architect specializing in complex puzzle mechanics and state management. Your task is to design the complete underlying logic system for a dynamic 2D grid puzzle based on the following rules.

Core Mechanic Description:
The system must manage a 6x6 grid where the primary interaction is based on element matching and chain reactions.

1. Trigger Condition: Connecting or grouping 3 or more adjacent elements of the same type triggers an "Explosion."
2. Explosion Effect: The explosion clears the triggered tiles and applies a localized force.
3. Post-Explosion Effect: The cleared area must immediately trigger a gravity-based tile drop and subsequent spawning of new tiles to fill the resulting gaps.

Required Output Structure:
Your response must exclusively detail the logical architecture. Do not include any visual styling, character descriptions, narrative, or game flow descriptions. Structure your output into the following three distinct sections:

1. 2D Grid Data Structure: Define the optimal data structure (e.g., array representation, hash map) used to manage the state of the 6x6 grid, including how element type, position, and state (filled/cleared) are stored.
2. Chain Reaction Algorithm (Recursive Logic): Provide a detailed, step-by-step recursive algorithm or function logic for detecting and processing chain reactions initiated by explosions. This must define how a single explosion propagates, handles simultaneous triggers, and manages state updates across the entire grid.
3. Gravity and Refill System Logic: Detail the specific mathematical or procedural logic governing the gravity-based tile drop and the spawning mechanism that fills the voids left by the explosions.

Ensure the logic is purely algorithmic and ready for direct implementation.