Submit the rebased + fixed + gated PR upstream

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What to build

A PR to JustVugg/colibri targeting dev that carries the rebase + shared-FFN fix + parity gate, CI green, following the house template (Summary / Validation / Compatibility) plus a "Correctness gate" section. Answer the one open design question the maintainer flagged for the cluster path — the cluster × expert-cache/RAM-budget interaction (JustVugg#403) — in one paragraph, pre-emptively.

Acceptance criteria

• PR targets dev (not main); CI green across the matrix; 0 warnings.
• Body has Summary / Validation / Compatibility plus a "Correctness gate" section stating byte-for-byte worker-vs-local equality, the oracle run twice (union ON and OFF), and "default path unchanged unless CLUSTER_WORKERS set".
• The RAM-budget/expert-cache interaction ([Bug]: generation RSS overshoots the projected peak by ~40 GB on GB10 (Grace 20-core, 121 GB unified) — three kernel OOM kills; a low --cap contains it, --ram does not JustVugg/colibri#403) is addressed in one paragraph.
• References the feat: add dense MLP activation sharding JustVugg/colibri#550 gate discipline the maintainer endorsed; enhancement label.
• If CI is held (action_required, first-time contributor), the maintainer is asked to approve the workflow run.

Environment: codegraph-indexed — run codegraph explore first; refs in /home/eli/dev/_reference-docs-and-repos/colibri.