The parser: equation strings to a term model

[roed-math]

The parser: equation string to ParsedEquation.

Recognizes the term model the rest of the classifier works with: polynomial
terms, 2^n-style exponential terms and y^q power terms, with parameters
living in the coefficient ring.

Denominators are cleared, and the hypotheses that introduces are kept as
structured, executable predicates rather than prose. API note:
ParsedEquation.conditions is a tuple of NonzeroCondition objects
(conditions.py), not a list of display strings; str(c) renders one and
c.as_dict() serializes it. The conditions are collected from the source
syntax before Sage can simplify a denominator away, so 1/(x - 1) = 1/(y - 1)
records x - 1 != 0 (not x != 0) and x/x = y keeps both the condition and
x as an unknown. source_lhs/source_rhs keep the user's equation beside
the normalized model, and is_solution() checks candidates against it exactly —
the final correctness filter for every solver's output.

No family data is involved: the parser is exercised by its own doctests and by
tests/test_parsing.py.

Part of the series that splits #1 into reviewable pieces:

1. 01-bibliography — packaging, docs, annotated bibliography
2. 02-parsing — equation strings to a term model
3. 03-registry — the YAML family registry (3 seed families)
4. 04-matchers — shape recognizers
5. 05-classify — the classification pipeline
6. 06-solvers — solver framework, two seed solvers, and the CLI
7. 07..09-backbone — the 23 parent families of the DAG, by depth
8. one PR per remaining family (38 of them, mutually independent)
9. 99-polish — restore the full doctests and tighten the invariants