feat(roadmap): add Modular Forms — Hecke theory, newforms, and L-functions

Grounded successor to the closed #36: keeps the classical arithmetic theory
(nebentypus → valence formula → Hecke algebra → Petersson → newforms → strong
multiplicity one → Atkin–Lehner/Fricke → L-functions → coefficient field is a
number field → LMFDB invariants); recasts the modular curve as the analytic
quotient Γ\ℍ with the dimension formulas by the valence route; and cuts the
geometric summit (Katz–Mazur moduli, algebraic Riemann–Roch, the GAGA bridge,
Galois representations, Jacobians) as out-of-scope for their own future roadmaps.

Layer 2 keeps the abstract double-coset Hecke ring and explains its adelic
specialization (the Shimura ↔ C_c(K\G/K) link), with the adelic construction
itself flagged as a future roadmap. Targets.lean keeps the four concrete
dimension instances and drops the two false free-parameter schemas.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: CBirkbeck

README.md

@@ -24,6 +24,7 @@ industry groups.
 10. [One-parameter semigroups, completely monotone functions, and BCR Bochner](TauCetiRoadmap/OneParameterSemigroups/README.md)
 11. [Exchangeability and de Finetti](TauCetiRoadmap/Exchangeability/README.md)
 12. [Weighted orthogonal L² bases: completeness, Hilbert bases, and products of orthogonal systems](TauCetiRoadmap/OrthogonalL2Bases/README.md)
+13. [Modular forms — Hecke theory, newforms, and L-functions](TauCetiRoadmap/ModularForms/README.md)
 
 ## Writing a roadmap