docs: quanticstransform missing documentation for error conditions, conventions, and examples

[shinaoka]

Description

Multiple documentation gaps were identified in tensor4all-quanticstransform by automated feature testing with 6 different user personas.

Missing documentation

1. No `# Errors` sections on any public function — users cannot discover error conditions without reading source
2. Fourier bit-reversal convention undocumented — QFT output uses bit-reversed ordering, which is not mentioned in quantics_fourier_operator docstring
3. Multi-variable indexing convention undocumented — interleaved bit encoding (s_n = bit_var0 + 2*bit_var1 + ...) for _multivar operators is not described anywhere in public docs
4. Index remapping workflow undocumented — the requirement to remap state indices to match operator get_input_mapping().true_index before calling apply_linear_operator is only shown in integration tests
5. Shift operator semantics misleading — docstring says f(x) = g(x + offset) but the matrix action is (Mg)[x] = g[x - offset]; the operator maps |x> -> |x+offset> which is the transpose direction
6. AffineParams column-major convention easy to miss — [[1, 2], [3, 4]] stored as [1, 3, 2, 4]
7. BinaryCoeffs(-1, -1) restriction unexplained — rejected but no explanation of why

Missing examples

• No examples/ directory for the crate
• No Python example for quanticstransform in docs/examples/python/
• No end-to-end example showing TCI -> create operator -> apply -> verify results

Inconsistencies

• "sites" vs "bits" terminology used interchangeably for parameter r

Found by

Automated feature testing of tensor4all-quanticstransform with 6 user personas.