[100 MRG] Feature: selection rectangular + fill/stroke #7

Mock selection state + fill/stroke; document live limits.

Claim (MergeOS MRG)

1. Follow https://github.com/mergeos-bounties
2. Star https://github.com/mergeos-bounties/mergeos
3. Star https://github.com/mergeos-bounties/mergeos-contracts
4. Comment: I claim this bounty
5. MergeOS Claim [25 MRG] Docs: GIMP 3 vs 2.10 batch flags matrix #1 with this issue link
6. PR to gimp-mcp master with Fixes #<n>

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