docs: add analytic frule/rrule formulas for scalar operations

[shinaoka]

Summary

The cases/ directory contains numerical oracle data (finite-difference and PyTorch references) for scalar operations, but lacks documentation of the analytic derivative formulas (frule/rrule) that implementations should follow.

Motivation

chainrules-scalarops in tenferro-rs is being extended to provide frule/rrule for all standard scalar operations. Having the canonical formulas documented here ensures:

• A single source of truth for derivative correctness across implementations
• Clear specification for complex-valued Wirtinger derivatives (∂f/∂z̄ convention)
• Easy review of edge cases (e.g., branch cuts, singularities at zero)

Scope

Add a document (e.g., docs/scalar-ad-formulas.md) covering at minimum:

Unary real/complex

op	primal	frule: df = ?	rrule: ∂L/∂x = ?
sqrt	√x	dx / (2√x)	ḡ / (2√x)
exp	eˣ	eˣ · dx	ḡ · eˣ
expm1	eˣ−1	eˣ · dx	ḡ · eˣ
log	ln x	dx / x	ḡ / x
log1p	ln(1+x)	dx / (1+x)	ḡ / (1+x)
sin	sin x	cos(x) · dx	ḡ · cos(x)
cos	cos x	−sin(x) · dx	−ḡ · sin(x)
tan	tan x	dx / cos²(x)	ḡ / cos²(x)
tanh	tanh x	(1−tanh²x) · dx	ḡ · (1−tanh²x)
asin	arcsin x	dx / √(1−x²)	ḡ / √(1−x²)
acos	arccos x	−dx / √(1−x²)	−ḡ / √(1−x²)
atan	arctan x	dx / (1+x²)	ḡ / (1+x²)
sinh	sinh x	cosh(x) · dx	ḡ · cosh(x)
cosh	cosh x	sinh(x) · dx	ḡ · sinh(x)
asinh	arcsinh x	dx / √(x²+1)	ḡ / √(x²+1)
acosh	arccosh x	dx / √(x²−1)	ḡ / √(x²−1)
atanh	arctanh x	dx / (1−x²)	ḡ / (1−x²)
conj	x̄	d̄x	ḡ̄

Binary

op	frule	rrule (∂L/∂x, ∂L/∂y)
add	dx + dy	(ḡ, ḡ)
sub	dx − dy	(ḡ, −ḡ)
mul	dx·ȳ + dy·x̄	(ḡ·ȳ, ḡ·x̄)
div	dx/ȳ − dy·x̄/y²	(ḡ/ȳ, −ḡ·x̄/y²)
powf	n·x^(n−1)·dx	ḡ·n·x^(n−1)
powi	n·x^(n−1)·dx	ḡ·n·x^(n−1)

Notes to include

• Complex convention: Wirtinger calculus with conj() on partial derivatives where needed
• Edge cases / singularities (sqrt at 0, log at 0, etc.)
• Relationship to existing oracle test data in cases/

Blocked by

• tensor4all/tenferro-rs chainrules-scalarops Phase 2a (adding the missing frule/rrule implementations)

Once the implementations are finalized, the formulas here should match exactly.