Fix runaway gem transmute loop: verify stack presence before every click

A "999" gem count (OCR couldn't read the digit, code assumes "convert
until depleted") relied on the loop noticing an empty stack and
breaking — but that check only ran when _gems_stack_monitor_for
returned None. For any registered gem type it always returns a static
screen coordinate, so the depletion check was dead code: the loop
just kept blindly clicking the same fixed position forever.

Observed in the wild: stuck on Topaz Flawless for 30+ minutes and 177
iterations (of a fake "999" target, ~2.7h worst case) before being
manually force-exited, repeatedly clicking fixed convert-panel/GEMS
coordinates with nothing real there — the likely cause of it also
grabbing and re-placing unrelated stash items during that time.

Now always does a live template search before clicking, breaking
immediately once the stack is genuinely gone, on every gem type.
Verified end-to-end with a mocked run: a fake depleted "999" stack now
stops instantly instead of looping, and a real gem right after it
still converts correctly.

src/transmute/transmute.py

@@ -879,14 +879,18 @@ class Transmute:
                 self._switch_to_gems_tab()
                 wait(0.3, 0.4)
 
-                pos = self._gems_stack_monitor_for(gem_template)
-                if pos is None:
-                    img = grab()
-                    matches = template_finder.search_all(gem_template, img, threshold=0.85, roi=left_roi)
-                    if not matches:
-                        Logger.info(f"  no {gem_template} left in GEMS tab -- stopping {tier_name}")
-                        break
-                    pos = matches[0].center_monitor
+                # Always verify the stack is actually still there before clicking it.
+                # A static coordinate from _gems_stack_monitor_for tells us WHERE to
+                # click but says nothing about whether anything is still there — for
+                # a "999 = depleted-unknown" plan (OCR couldn't read the count), that
+                # made the depletion check dead code: it kept blindly clicking the
+                # same empty slot every iteration instead of ever stopping.
+                img = grab()
+                matches = template_finder.search_all(gem_template, img, threshold=0.85, roi=left_roi)
+                if not matches:
+                    Logger.info(f"  no {gem_template} left in GEMS tab -- stopping {tier_name}")
+                    break
+                pos = self._gems_stack_monitor_for(gem_template) or matches[0].center_monitor
 
                 # Step 1: ctrl+shift+left-click x3 -- 3 gems move from GEMS tab into the convert panel.
                 Logger.info(f"  [{i+1}/{n_transmutes}] loading 3x {gem_template} into GEMS convert panel @ {pos}")