feat: widen pool source to per_entity_per_period facts

[mohossam01]

What this PR does

Register scalar and vectorized dispatch handlers for pool.<attr> columns on per_entity_per_period fact tables — the most common fact grain, one row per entity per period. Each handler reads the row's entity from context and draws uniformly from its declared pool; the load-time pool-coverage validator widens to admit the new grain. A column like payment_type: pool.payment_method now coexists with SCD2 and CDC on a single transactional fact. Per_period facts, reference dims, and sub-entity dims stay out of scope — those rows have no per-row entity binding.

Files Modified

• plotsim/tables.py
• plotsim/validation.py
• tests/test_pool_on_fact.py
• tests/test_pool_attr.py
• docs/site/column-types.md
• docs/site/feature-reference.md
• CHANGELOG.md

How to test

python -m pytest
python -m mkdocs build --strict