ci: build master changes through a merge queue

[joaodinissf]

Why the change

Master should only ever move to a commit that was built exactly as the snapshot build will build it, so two PRs that are green on their own can no longer break master together or share a version.

Special things to note

• Needs a repository admin after merging: add a merge queue rule to the master ruleset, with rebase merges, one PR per group, one group building at a time, all-green grouping and a 60-minute check timeout. Adding it first would leave queued PRs waiting for checks that never run.
• Merge-queue builds use the published snapshot like the snapshot build does, and fail on a mismatch. -Dtycho.baseline=failCommon fails the build when an artifact has a version the published snapshot already has, with different content. Tycho's default only warns and swaps in the published copy. On 28 Sep, fix(xtext): match case-sensitive pattern lookups against the candidate name #1551 and fix(xtext.ui): iterate a snapshot of the matches when showing a running search #1552 were merged 9 s apart, so the runtime feature got one version for two contents. The snapshot build swapped in fix(xtext): match case-sensitive pattern lookups against the candidate name #1551's feature, which pins xtext.ui 17.3.3, and failed to resolve. The last good snapshot swapped 36 artifacts, all identical, so the guard has no false positives there. Because the queue builds one PR at a time and each takes a full CI run, two merges can no longer land in the same minute.
• PR builds no longer read the published snapshot (-Dtycho.baseline=disable -Dtycho.baseline.replace=none). A PR build only has to show that the PR is correct; the queue checks it against master. Today one bad snapshot fails every open PR, and so does a PR whose commits are older than master's last change to the same feature. The release version check (compare-version-with-baselines) is unchanged.

Change outline

 .github/workflows/verify.yml
+  on: merge_group
   maven-verify:
+    pull_request → -Dtycho.baseline=disable -Dtycho.baseline.replace=none
+    merge_group  → -Dtycho.baseline=failCommon
 .github/workflows/snapshot.yml
+  -Dtycho.baseline=failCommon

flowchart LR
  PR[PR build: no published snapshot] -->|approved + auto-merge| Q[merge queue: one PR at a time]
  Q --> B[queue build = snapshot build<br/>swap + failCommon]
  B -->|green| M[master fast-forwards]
  B -->|red| X[PR leaves the queue, master untouched]
  M --> S[snapshot build publishes]

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