docs: Document public API for customizing gravel sieve ore probabilities

[Copilot]

Users had no way to discover that gravelsieve exposes a public API for excluding or adjusting ore drop probabilities, requiring code inspection to find it.

Changes

• README: Document the gravelsieve.set_probabilities() API, including how to exclude specific ores by setting their probability to 0

Usage

From a dependent mod (declare gravelsieve in mod.conf):

gravelsieve.set_probabilities({
    ["default:gravel"] = {
        ["default:gold_lump"] = 0,   -- excluded
        ["default:diamond"] = 0,     -- excluded
    }
})

Remaining probabilities are automatically normalized. The call is safe to make at load time — all overrides are merged after all mods have loaded, so load order doesn't matter.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>excluding ores from the gravel sieve</issue_title>
<issue_description>is there a way I can easily exclude ores from being able to be sieved?</issue_description>

<agent_instructions>Can you answer this. My guess the minetest.after(2,... is after the list output</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@joe7575 Yes, this is possible without modifying the gravelsieve code! The mod exposes a public API for this.

From your own mod (which should list gravelsieve as a dependency in mod.conf), call:

gravelsieve.set_probabilities({
    ["default:gravel"] = {
        ["mymod:ore_i_want_excluded"] = 0,
        -- add more ores to exclude here
    }
})

Setting an ore's probability to 0 effectively excludes it from sieving results. The remaining probabilities are automatically normalized.

This works because gravelsieve.set_probabilities() collects all overrides and merges them after all mods have loaded, so load order doesn't matter as long as the dependency is declared.

@joe7575 No, the standalone [Minetest-Gravelsieve](https://github.com/joe7575/Minetest-Gravelsieve) does not have the `set_probabilities()` API — that was added later in the techpack version.

However, the gravelsieve.ore_probability table is global, so you can still remove ores from another mod. You just need to do it after the ore table is populated (which happens via minetest.after(2, ...)):

-- In your own mod's init.lua (with gravelsieve as dependency):
minetest.after(2, function()
    gravelsieve.ore_probability["default:gold_lump"] = nil
    gravelsieve.ore_probability["default:diamond"] = nil
    -- remove any other ores you want to exclude
end)

Setting the entry to nil removes the ore entirely from possible sieve results.</comment_new>

• Fixes excluding ores from the gravel sieve #135

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