load_haxby_example: option to share the block order across runs

`load_haxby_example` draws a fresh random block order for every run (`rng.permutation(_HAXBY_CONDITIONS)` per run), so intersubject correlation across runs treated as subjects is zero by construction until the TRs are re-sorted into one shared condition sequence. The quickstart does that with a small helper on the page. A keyword such as `block_order="shared"` (default unchanged) that draws the order once and reuses it for every run would let ISC, ISRSA and alignment examples use the runs directly. Small; keep the return contract.

Start at the `load_haxby_example` entry point and read the quickstart helper that currently re-sorts TRs into a shared condition sequence. Trace how the block order is generated for each run and preserve the existing return contract. Done means an opt-in shared order is reused across runs while the default behavior remains unchanged, supporting direct ISC, ISRSA, and alignment examples.

Written by the indexing model from the issue text.