Wire animated block rendering

[newmattock]

Addresses OpenSword#1
References OpenSword#3

Completes the remaining animation hookup and cleanup slice discussed in OpenSword#1 and OpenSword#3.

Changes:

• Wire IsometricRenderer to use BlockType.get_current_frame(animation_time) so animated block types actually advance frames when animated and animation_frames are configured.
• Track animated blocks separately and update their sprite textures from _process() without redrawing the whole world.
• Configure the default water block with two frames and add a visible foam-frame SVG so the animation path has a concrete in-demo block to exercise.
• Fix startup chunk loading so the first full render actually displays the generated world instead of clearing block sprites before any chunks are marked loaded.
• Add a visible starter pond near the opening camera position so the demo immediately shows the animated water path.
• Remove the unused renderer texture-generation helper and unused tile-size constants.
• Keep the cursor highlight z-index inside Godot's supported canvas range so the demo scene starts without a runtime z-index error.
• Update API/debugging docs so they match the current InteractionMode enum and no longer describe a nonexistent QUERY mode.

Validation:

• git diff --check
• rg -n "QUERY|_create_colored_texture" scripts docs returns no matches
• godot --version -> 4.6.2.stable.official.71f334935
• godot --headless --path . --import
• godot --headless --path . --scene res://scenes/Main.tscn --quit-after 180
• godot --path . --scene res://scenes/Main.tscn --write-movie /tmp/openisopix-demo.avi --quit-after 180
• godot --path . --scene res://scenes/Main.tscn --write-movie /tmp/openisopix-frames/frame.png --fixed-fps 30 --quit-after 60

Local demo evidence:

• Recorded a Godot movie: 180 frames at 60 FPS, 1280x720, SHA-1 643e9af3851abb0ed507bb34a9153e9eac96c046.
• Generated a five-frame representative PNG sample ZIP from a 60-frame Godot recording, SHA-1 6071dfa10937b4f9a2b5c555f72724bd791e17b2.
• Sampled frame hashes alternate between c04c718e7ea533cc9d7d674416c8f084541963f5 and cb86b0b945bc1dbb1b0653ca6b8c4b8bd02ab977, matching the configured water animation frames.