Make an advanced powder toy lua mod that adds a single pythonic particle that acts like a solid, it's really durable and has uni-frame, and subframe regeneration. It'll have an interesting special property that'll expand inwards when a frame of itself is damaged.

Reference Script (REFERENCE ONLY):

```
-- Allocate a new element for the durable solid with subframe regeneration
local subr = elements.allocate("custom", "SUBR")
elements.element(subr, elements.element(elements.DEFAULT_PT_METL))  -- Base on METL (metal)

-- Properties of the new element
elements.property(subr, "Name", "SUBR")
elements.property(subr, "Description", "Durable solid with subframe-super-regenerative mechanic and weird conductive properties.")
elements.property(subr, "Color", 0xFF5500)  -- Orange color to represent the solid material
elements.property(subr, "Weight", 50)       -- Lightweight but solid
elements.property(subr, "Hardness", 500)    -- Highly durable
elements.property(subr, "HeatConduct", 200) -- High heat conductivity
elements.property(subr, "Temperature", 295.15) -- Room temperature

-- Custom properties for subframe regeneration
local REGEN_RATE = 90   -- Percentage chance of subframe regeneration
local REGEN_RADIUS = 2  -- Regeneration affects neighboring particles within this radius

-- Unique conductive properties
local CONDUCT_RANDOMNESS = 50 -- Random chance for conduction effects
local CONDUCT_ENERGY = 200    -- How much energy is conducted randomly

-- Update function for the subr element
local function update_subr(i, x, y, s, n)
    -- Subframe regeneration mechanic
    if math.random(0, 100) < REGEN_RATE then
        sim.partProperty(i, "life", 999) -- Max out life (indicating healing)
        
        -- Regenerate nearby elements rapidly
        for r in sim.neighbors(x, y, REGEN_RADIUS, REGEN_RADIUS) do
            if sim.partProperty(r, "type") ~= subr then
                sim.partChangeType(r, subr) -- Convert nearby elements to subr
                sim.partProperty(r, "life", 999) -- Also heal them fully
                sim.partProperty(r, "dcolor", 0xFFAA00)  -- Change color to bright orange during regeneration
            end
        end
    end

    -- Weird conductive properties
    if math.random(0, 100) < CONDUCT_RANDOMNESS then
        -- Randomly transfer energy/heat to neighboring particles
        for r in sim.neighbors(x, y, 1, 1) do
            if sim.partProperty(r, "type") ~= subr then
                -- Randomly increase/decrease temperature and pressure for weird effects
                local new_temp = sim.partProperty(r, "temp") + math.random(-CONDUCT_ENERGY, CONDUCT_ENERGY)
                sim.partProperty(r, "temp", new_temp)

                -- Randomly increase/decrease velocity
                local vx = sim.partProperty(r, "vx") + math.random(-2, 2)
                local vy = sim.partProperty(r, "vy") + math.random(-2, 2)
                sim.partProperty(r, "vx", vx)
                sim.partProperty(r, "vy", vy)

                -- Optionally cause the element to change type due to weird conduction
                if math.random(0, 100) < 5 then
                    sim.partChangeType(r, elements.DEFAULT_PT_PLSM) -- Change to plasma randomly
                end
            end
        end
    end

    -- Visual feedback: change color based on temperature/conductivity activity
    local temp = sim.partProperty(i, "temp")
    if temp > 500 then
        sim.partProperty(i, "dcolor", 0xFF0000)  -- Turn red when really hot
    elseif temp < 273 then
        sim.partProperty(i, "dcolor", 0x0000FF)  -- Turn blue when cold
    else
        sim.partProperty(i, "dcolor", 0xFF5500)  -- Default orange
    end
end

-- Assign the update function to the new element
elements.property(subr, "Update", update_subr)

-- Enable the element to be solid and not flow like powder
elements.property(subr, "State", elements.ST_SOLID)

-- Make the element indestructible unless melted or extreme forces applied
elements.property(subr, "Explosive", 0)  -- Not explosive