docs: refresh the container image sizes for the 2026.07 images, and keep them in one place

[mmcky]

#199 moved both images to the Anaconda 2026.07 baseline. The docs still quote the 2026.06 sizes, which containers/README.md now dates as measured before the move. The image-size job in test-container.yml measured the new images on 2026-09-25 (run 36122900672):

Image	Compressed pull	On disk
quantecon (full)	3.39 GB (was 3.33)	8.79 GB (was 8.60)
quantecon-build (lean)	2.85 GB (was 2.93)	7.10 GB (was 7.32)

The lean image is now about 16% smaller to pull than the full one, not the ~12% containers/README.md:12 says.

The figures are hard-coded in 22 lines across 10 files, so every baseline or TeX Live change leaves most of them stale:

File	Lines
README.md	153, 154
containers/README.md	9, 10, 12
containers/quantecon-build/README.md	18, 79, 80
docs/CONTAINER-GUIDE.md	9, 10, 64
docs/QUICK-REFERENCE.md	23, 24
docs/dev/ARCHITECTURE.md	15, 54
docs/dev/CONTAINER-VALIDATION.md	17, 18
docs/dev/TESTING.md	187
setup-environment/README.md	34, 35
.github/copilot-instructions.md	33, 34, 135

• Choose: one canonical size table (containers/README.md already states the method and the date) that the other docs link to, or refresh every mention in place.
• Update the numbers, the measurement date, and the "~12% smaller to pull" comparison.

Deferred here from Copilot's review of #199; the replies on that PR point at this follow-up.

Assisted-by: Claude Code (Claude Opus 5.5)