test: add first word-level reading measures tests

[shaitarAn]

This PR adds the first unit tests for word-level reading measures.

Measures covered:

• TFC – Total Fixation Count
• TFT – Total Fixation Time
• FFD – First Fixation Duration
• FD – Fixation Duration (per-word aggregate)
• FPRT – First-Pass Reading Time
• FRT – First Reading Time
• FPFC – First-Pass Fixation Count
• FPF – First-Pass Fixation (flag)
• SFD – Single Fixation Duration
• RRT – Regression Reading Time
• RR – Regression indicator

Scenarios tested:

• Single fixation on a word
• Multiple fixations within first pass
• Skipped word (no fixations)
• Late regression (return to a previously read word)
• Early regression with multiple rereading fixations
• Fixation on space character mapped to word AOI