Document upstream rebase process in PORTING.md

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Goal

Document the upstream rebase process so that any contributor (including Claude Code) can perform it without guesswork.

Context

compositor is a fork of cosmic-comp. Upstream is actively developed by System76.
Weekly upstream merges are the goal. Without documented process, each rebase risks inconsistency.

Scope

Extend PORTING.md with a dedicated section covering:

Rebase procedure

1. Fetch upstream: git fetch upstream
2. Create rebase branch: git checkout -b chore/rebase-YYYY-MM-DD
3. Rebase: git rebase upstream/master
4. Known conflict zones (always expect conflicts here):
   • src/state.rs (Common struct, new() initializer)
   • src/wayland/protocols/mod.rs (new protocols upstream adds)
   • src/wayland/handlers/mod.rs
   • Cargo.toml (dependency versions)
5. Resolution strategy per conflict zone
6. Verify: cargo check --lib passes
7. Verify: cargo test --lib passes
8. Document any new upstream changes in PORTING.md

What CI checks

• cargo check --lib on every push
• cargo test --lib on every push
• Upstream rebase CI (what it does and how to interpret failures)

PORTING.md current Lunaris changes

Document every intentional deviation from upstream with rationale:

• xdg-decoration: always ClientSide
• shell_overlay protocol registration
• event_bus integration
• cosmic-protocols kept (reason: merge conflict avoidance)

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PORTING.md updated with all Phase 3 changes. Needs another pass to document: IcedElement removal, window_header protocol, zoom protocol, LunarisTheme migration, libcosmic removal.