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feat: expose the building blocks of the bound computations (#126)

`compute_forward_bounds` and `compute_backward_bounds` are thin wrappers that
combine a norm function and a light cone with `compute_bounds`. Only
`compute_bounds` was public, so a custom bound could not be assembled without
reaching into private modules. The pieces the wrappers use are now exported
from `qiskit_addon_slc.bounds` and documented:

- `time_evolved_norm_forward`, which bounds a single Pauli error term evolved
  forward to an observable;
- `time_evolved_norm_backward`, previously `_time_evolved_norm_backward`, which
  bounds a single Pauli error term evolved backward to the all-zero initial
  state, an assumption its docstring now states;
- `LightCone`, whose docstring now explains how it is seeded with the operator
  being bounded against, and that `commutes` mutates it, so a fresh one is
  needed for every call to `compute_bounds`.

`time_evolved_norm_forward` defaulted `eigval_max_qubits` to the largest
unsigned integer, so called on its own it always attempted the eigensolver,
however wide the commutator. It now shares the default of 14 with
`compute_forward_bounds` through one constant, so bounding a single term agrees
with bounding a whole circuit.

Also adds the missing space in the warning `merge_bounds` logs when it falls
back to partitioning the list of noisy boxes.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

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