fix: #3613480 Ship high-res sources for soft demo photos and correct stale media dimensions

Closes / relates to #3613480.

Two demo-media corrections, both surfaced after the hi-res-hero swap in MR !11 (merged):

1. Ship genuine high-resolution sources for 3 soft demo photos The 3 photos below still shipped small legacy sources stretched into larger display boxes (rendered at 200 but soft/upscaled). Replaced each with its true-resolution original from the Educare stylesheet, cropped to the SAME aspect ratio as before (framing unchanged), downscaled ~2x the display box:

• two-students-holding-books-in-the-library.jpg 652x500 -> 1700x1304
• three-students-with-textbooks.jpg 669x492 -> 1500x1103
• speaker-presenting-at-a-campus-event 1140x492 (PNG) -> 2640x1140 (JPEG; png removed)

2. Correct stale media dimensions after the earlier hi-res swap MR !11 (merged) upgraded 6 photos to hi-res but left the old small field_media_image width/height recorded. drimage caps derivatives at the recorded source dims, so heroes stayed capped (the news-article _1320_743 hero was the broken-hero case). Corrected the 6 media entities to the real pixel dimensions:

• students-writing-in-a-classroom 285x184 -> 1600x1033
• students-walking-outside-the-science-building 1140x492 -> 2731x1179
• two-students-talking-on-a-campus-path 669x492 -> 1600x1177
• student-studying-at-a-library-computer 455x238 -> 2640x1381
• student-studying-at-a-computer 455x238 -> 2640x1381
• students-high-fiving-in-the-library 285x184 -> 1125x726

Verified on two independent fresh installs (drush site:install recipes/educare): all 9 images serve crisp hi-res derivatives (derivative width >= display box), 0 broken images sitewide, alt/title unchanged. Green-gated locally with gitlab-ci-local (all jobs pass).

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