ci: give every push to next its own CI result

[altaywtf]

Change

Every push to next keeps its own Next CI run, so each merged commit gets its own result.

Event	Before	After
Push to next	one shared group; a new merge cancelled the running run	a group per run; nothing cancelled
Pull request push	a group per ref; the stale run cancelled	unchanged, as in putio-android

• Risk: back-to-back merges no longer cancel each other, so they queue for the organization's macOS runners.

Validation

• pnpm run verify: the new concurrency test fails against the old settings and two near-miss variants
• actionlint on ci-next.yml
• Unverified: mise run verify not run locally; no Swift or harness input changed.

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Written by an agent (Claude Code, Opus 5.5)

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