riemann-roch (#28): Riemann–Roch (namespace RS)

API summary (see docs/design/serre-duality-tails.md §9.1). Builds on serre-duality-tails (BUILT, including its chiT-ledger closure — TailDuality.chiT_eq_chiT_zero_add_degree, the one prerequisite this pass discharged). Unit COMPLETE: zero sorries, scripts/check.sh Jacobian/RiemannRoch passes. NOT registered in Jacobian.lean (orchestrator's job).

A thin assembly unit — every proof is omega-level ℤ-arithmetic over TailDuality's tail ledger and Serre-duality export bank; no new mathematics, no reference to T D/pairT/chi's internals.

Exports (all in namespace RS)

• chiT_zero : RS.TailDuality.chiT (0 : RS.Divisor X) = 1 - (genus X : ℤ).
• riemannRoch {ω₀ : RS.MForm X} (h₀ : ω₀ ≠ 0) (D : RS.Divisor X) : (RS.l D : ℤ) - (RS.l (RS.canonicalDivisorOf ω₀ - D) : ℤ) = D.degree + 1 - (genus X : ℤ) — Riemann–Roch.
• l_K_eq_genus {ω₀} (h₀) : RS.l (RS.canonicalDivisorOf ω₀) = genus X.
• deg_canonical {ω₀} (h₀) : (RS.canonicalDivisorOf ω₀).degree = 2 * (genus X : ℤ) - 2.
• riemann_inequality (D : RS.Divisor X) : (D.degree + 1 - (genus X : ℤ)) ≤ (RS.l D : ℤ) — unconditional (no reference form needed); the forward-headline seed genus-zero-headline (#30) consumes directly, at D := single P 1 under genus X = 0.

Consumer notes

• cech-h1-genus (#27): does not need anything from here — it re-exports TailDuality.h1T_zero_eq_genus directly.
• genus-zero-headline (#30): consumes riemann_inequality (forward direction: genus 0 ⇒ l(single P 1) ≥ 2, extracting a function with a single simple pole).