Fix auto-platforms absorbing bombs and scaling with catcher_width

[jherrflexion]

Summary

• Bomb coins now fall through auto-catcher platforms so only the player's catcher can detonate them
• Auto-platforms stay at a fixed 60 px width regardless of the catcher_width upgrade (which was unintentionally widening every auto-platform alongside the player catcher)
• Removed a now-unused upgrade_purchased signal subscription and a dead _base_scale field

Test plan

• godot --headless --script res://tools/lint_project.gd — all 8 scenes OK
• godot --headless --script res://tools/run_tests.gd — 235/235 pass
• validate-all / validate-ui — 0 issues
• Bomb spawned directly above auto-platform with player moved off-center → currency unchanged (no trigger_bomb)
• Silvers scattered across patrol zone with player off-center → currency gained from auto-platform catches
• 10 levels of catcher_width upgrade → player catcher grew 100→250 px, auto-platform sprite scale stayed at exactly 0.46875 (60 px)
• Bomb on player catcher still triggers trigger_bomb (regression check)

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