ci(e2e): isolate concurrent integration rounds

[drew]

Summary

Keep integration and conformance jobs alive when up to five E2E stability workflows run on one branch. This PR is stacked on #3586, above #3584 and #3583.

Related Issue

No issue required: this is a localized CI orchestration fix found in the first diagnostic stability wave.

Changes

• Give integration runner calls a run-specific concurrency key during manual stability campaigns.
• Preserve the existing cancellation behavior for normal PR and merge-queue runs.
• Pass stability mode to all three integration categories and document the behavior.

Testing

• mise run pre-commit passes.
• Actionlint passes for both changed workflows.
• Ten-round stability campaign on this PR's SHA is pending.

Checklist

• Follows Conventional Commits.
• Commit is signed off (DCO).
• Architecture docs updated.

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[drew]

Closing to narrow the automated flake investigation to the four selected fixes (#3583/#3588, #3598, #3604, #3608). The findings remain available here for later follow-up.