feat: make the store-screenshot normaliser form-factor aware

Play gives phones and each tablet size a screenshot slot of their own (Phone, 7-inch
tablet, 10-inch tablet) and asks for at least four screenshots per large-screen form
factor, so the normaliser takes FORM_FACTOR=phone|tablet-7in|tablet-10in and keeps one
raw and one output directory per factor:

  store-assets/raw-screenshots/<form-factor>/   adb captures (git-ignored)
  store-assets/screenshots/<form-factor>/       what gets uploaded

The tablet form factors crop 0 px of system chrome on purpose, and that is checked
rather than guessed: every tablet profile in the SDK's own device definitions is already
inside Play's 2:1 cap (7in WSVGA 1024x600, 10.1in WXGA 1280x800, Nexus 7 2013
1200x1920, and Nexus 10 / Medium Tablet / Pixel Tablet at 2560x1600), so the 93/130 px
phone crop exists only because a 1080x2400 phone frame is 2.22:1. Cropping a tablet is
therefore cosmetic, and is now opt-in through STATUS_BAR_PX / NAV_BAR_PX.

The 2:1 clamp still runs for every form factor, and each file is now checked against
Play's limits as it is written (shorter side >= 320 px, longer side <= 3840 px) - the
phone captures only ever got that check by hand. A missing input directory or an unknown
form factor now fails with an actionable message instead of a bash error.

The four existing phone captures move to raw-screenshots/phone/ and normalise to the
same 1080x2160 RGB files as before, byte for byte. The 7-inch and 10-inch captures were
taken with this script on AOSP AVDs; how, and the numbers they show, are recorded in
docs/STORE_LISTING.md.

tools/make-store-assets.sh now prints the two tablet invocations as well, so the obvious
next step after generating the icon and feature graphic is not the phone-only one.

tools/make-store-assets.sh

@@ -38,4 +38,6 @@ render() { # <svg> <width> <height> <filename> <keep-alpha|flatten>
 echo "Writing store assets to $out"
 render "$here/store-assets/icon.svg" 512 512 icon-512.png keep-alpha
 render "$here/store-assets/feature-graphic.svg" 1024 500 feature-graphic-1024x500.png flatten
-echo "Done. Screenshots: ./tools/make-store-screenshots.sh   # reads store-assets/raw-screenshots/"
+echo "Done. Screenshots: ./tools/make-store-screenshots.sh   # reads store-assets/raw-screenshots/phone/"
+echo "      Tablets:     FORM_FACTOR=tablet-7in ./tools/make-store-screenshots.sh"
+echo "                   FORM_FACTOR=tablet-10in ./tools/make-store-screenshots.sh"