fix(wayland): retain output modes until finished

[eaceaser]

Summary

Retain zwlr_output_mode_v1 proxies until their Wayland lifecycle ends instead of releasing them immediately when announced.

Motivation

Commit 489de6f76 introduced immediate mode-proxy release while adding monitor serial-number support.

A compositor may reference an announced mode in subsequent events such as current_mode. On Niri, this caused libwayland to abort during startup:

unknown object (...), message current_mode(o)

Mode proxies are now tracked independently from output heads and released when:

• The mode emits finished
• The output manager finishes
• The Wayland connection is cleaned up

Independent tracking avoids relying on cross-object ordering between mode and head finished events.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

N/A

Testing

meson setup build-debug --wipe -Dtests=enabled --buildtype=debug
meson compile -C build-debug
meson test -C build-debug --print-errorlogs
just format

All 68 tests passed. The original startup crash was reproduced, and initialization completed successfully with mode proxies retained on Niri 26.04.

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Not applicable; this change has no visual or UI behavior.

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

[ItsLemmy]

Thanks a lot, I got hit by the bug and started fixing before seeing your PR.
I'll keep a simplified mix of versions.