docs: plan shared algebraic reflection

[kim-em]

Summary

• specify a strict copy-switch-delete migration of reusable algebraic machinery from Grind into Lean.Meta.Sym.Arith before any generalization
• describe the later SymM standalone interface and fixed algebraic views while keeping Lean syntax recognition closed
• specify the Mathlib-free Hex reflection, provider, matrix, factorization, rational-expression, and Grind integration architecture
• define the boundary and companion libraries for Mathlib-facing support

Validation

• git diff --check origin/main...HEAD
• documentation style and relative links checked locally
• no Lean build run because this PR changes planning documents only