[lorenzocerrone]

Fixes #277.

axes_order kept any unresolvable name as a virtual (singleton) axis, so a typo ("czyq") became a phantom dimension with x dropped, and the failure surfaced much later in the caller's own code.

• A name that is not on disk is accepted only if it is a canonical axis (t, c, z, y, x) or the name the AxesSetup maps to one; it normalises to the canonical name. "czyx" on a c,y,x image still gives (1, 1, y, x).
• Any other name raises NgioValueError naming it, the full order, the image axes and the canonical set.
• The duplicate-axis error names the repeated axis and echoes the order.
• New validate_axes_order(dimensions, axes_order) in ngio.io_pipes._ops_axes; the five iterator constructors call it, so a bad order fails at construction rather than at the first read.
• The existing container test asked for a "virtual" axis; it now asks for "z", keeping the coverage.
• One sentence in the images and iterators getting-started pages.

Checklist before merging

• I added an appropriate entry to CHANGELOG.md

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