feat: explore pure PAY API wrapper

[Mjboothaus]

Summary

• add experimental side-effect-free PAY(...) support in the symbolic parser via an identity wrapper
• keep LOGPAY(...) unsupported and explicitly rejected for core pricing paths
• document OpenSourceRisk-inspired PAY exploration in the README
• add parser and symbolic problem tests for PAY support and LOGPAY rejection

Validation

• uv run ruff format src/pdealchemy/math_bridge/parser.py tests/math_bridge/test_parser.py
• uv run ruff check src/pdealchemy/math_bridge/parser.py tests/math_bridge/test_parser.py
• uv run ty check src
• uv run pytest tests/math_bridge/test_parser.py tests/test_cli.py

Context

• Conversation: https://app.warp.dev/conversation/79aa7721-1947-44e5-840f-d20883bd8624
• Plan artifact: https://app.warp.dev/drive/notebook/eZQFltrUPlV50R8ZwHEybR

Co-Authored-By: Oz oz-agent@warp.dev

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
README.md	Unsupported file format
src/pdealchemy/math_bridge/parser.py	0% smaller
tests/math_bridge/test_parser.py	0% smaller