The front page's picture check stops growing: it keeps only the pictures it shows #866

Closes #768

What changes. strangersThumb in actions/front.ts remembered, per picture path, whether a thumb was taken as a stranger, and never forgot. Every retaken picture added an entry for the life of the process.

Now the memory is per forge (WeakMap<App, …>) and is pruned on every page: frontPage asks about the paths it shows, then forgets every path it did not ask about. The memory is therefore never larger than the current pictures, however often they are retaken. A picture still on the page is read once, as before.

There is no test-only export (the hollow audit holds).

Test. test/front.test.ts, by behaviour, with a spy on keepLook:

• a picture already checked is not read again;
• each of 25 retakes reads its new picture exactly once;
• showing the first path again reads it afresh, which proves it was forgotten rather than kept forever.

Local runs: front, studio and the cross files give 274 pass. tsc is clean.

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