fix(outliner): multi-group concurency on same object

[Kurtil]

Description

This fixes an outline pass issue where outlined tile proxies were keyed only by tile mesh. When the same tile was outlined in multiple groups, one group could overwrite or detach the other, causing non-deterministic outline behavior (see first screen recording where wall items lose their outline).

Root cause

Outlined proxies were stored with a tile-only key, while group operations (update/clean/remove) expected per-group ownership.

Additional context

Before:

BrokenOutline.mov

After:

workingOutline.mov

Note that on this second video, the example is slightly updated to make the select outline group higher priority than the wall outline group.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following:

• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Follow the Conventional Commits v1.0.0 standard for PR naming (e.g. feat(examples): add hello-world example).
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.