Preserve sources of resistances granted to minions

[AdamZ-8113]

Fixes #10241

Description of the problem being solved:

Bonemeld's added minion resistances show as coming from Player in Calcs, with a blank Source Name.

This is because the calculation discards the original modifier source when creating the minion resistance bonuses. This preserves each contributing source so Calcs can show its name and the existing item tooltip.

Each contribution is now rounded down separately. A single source like Bonemeld will be accurate, but multiple sharing modifiers can produce slightly different totals: two 50% contributions from 75% player resistance give 37 + 37 = 74 instead of 75. The alternative solution would be to combine sources back to a generic Player to prevent small rounding errors.

Steps taken to verify a working solution:

• Added save/reload regressions for Bonemeld attribution and multiple resistance-sharing sources, including positive and negative rounding.
• Verified expected behavior using Bonemeld and a separate custom modifier.
• Passed all tests

Link to a build that showcases this PR:

https://pob.codes/b/mpx-NPDYaIq

Before screenshot:

After screenshot: