fix(ddx-core): divide a quotient's derivative by its denominator once

[alxmrs]

Stacked on the #87 PR. Found by the v2 soak (#92), which had to exempt infinite data because of it, and refined after a tiny-values soak.

The textbook quotient rule (du·v - u·dv)/v² squares v:

• at v = ∞ (a value in the data) it evaluates ∞/∞ = NaN where the limit is 0;
• at a tiny v (1e-163) v² underflows to 0.

ddx now emits (du - (u/v)·dv)/v, which divides by v once, through the quotient itself. That's the same number everywhere else, finite at v = ∞, and it never squares v.

I first tried splitting it into du/v - u·dv/v². That handles v = ∞, but at a tiny v its two 1/v terms cancel to noise: u/sinh(u) differentiated to ±1e159 instead of about 0. When the denominator doesn't depend on the variable, the result is just du/v.

Rendered derivatives change accordingly: d/dx x/y is CAST(1.0 AS DOUBLE) / y. The JAX oracle suites (137 tests), ddxdb's tests, and a 5-minute soak (227k cases) pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CvszJhH8pn8H9G69wEPMU2

[alxmrs]

😈🧪 Adversarial tester: ✅ approved for correctness. Reviewed at d2c7bdc (identical tree to today's rebase febfc15). Workspace tests, doctests, a 20-min soak (0 failures) and a mutation spot-check all pass. The one blocker in the stack is on #79 (#79 (comment)); the full report is in #102 (comment).

[alxmrs]

LGTM, good catch.