Add benchmark to Flash README

[rejojer]

Adds a Benchmark section to the Flash package README: two figures, a token
table, and the measurement conditions.

Nine PDFs from 9 to 1,098 pages, each run end to end with tree optimization.
Time and cost both scale close to linearly with document length. The two things
the fit line does not show are called out in the text: cost tracks node count a
little more closely than page count, and wall clock flattens past roughly 700
pages where summary concurrency rather than length becomes the limit.

Figures live in pageindex/flash/assets/ so they travel with the package. These
are the first tracked images in the repo; the top-level README uses external
URLs instead.

The numbers are tied to the model and concurrency they were measured with
(gpt-5.6-luna, priced cold, no prompt-cache discount), so they will need a
re-run if either changes.