Spatial sorting

[Cubix33]

Closes #168

📝 Description

This PR refactors the PDF annotation sorting logic in src/filler.py. It replaces the strict coordinate-based sort with a Y-Cluster Spatial Sorting algorithm. This ensures that form fields on the same visual line (like "First Name" and "Last Name") are grouped correctly into "rows" before being sorted left-to-right, even if their Y-coordinates differ by a few pixels.

🛠️ Technical Changes

• Y-Cluster Implementation: Introduced a clustering mechanism in src/filler.py that groups PDF widgets within a 10-pixel Y-axis tolerance.
• Two-Pass Sorting:
  1. Vertical Pass: Groups annotations into "visual rows" based on their bottom-edge Y-coordinate (Rect[1]).
  2. Horizontal Pass: Sorts each individual row strictly by the X-coordinate (Rect[0]).
• Robustness: This prevents "Last Name" from being filled before "First Name" simply because the box was drawn 1 pixel higher on the page.

💡 Rationale

Strict coordinate sorting often fails on professionally designed, multi-column forms or tables where fields are meant to be read left-to-right but may have slight misalignments in the underlying PDF code. This change allows FireForm to handle dense, complex layouts (medical, legal, and government forms) with much higher accuracy.

🧪 Quality Check

• Verified that side-by-side fields on the same visual line are filled correctly (Left → Right).
• Confirmed that the vertical (Top → Bottom) flow is maintained for multi-row forms.
• Validated that the 10-pixel tolerance threshold effectively handles common PDF alignment variances.
• Fallback tested on standard single-column PDFs to ensure zero regressions.