e2e: remove activator container and consolidate to onchain allocation

[elitegreg]

Summary

• Bundles activator-removal phase 2.1, 2.2, and 2.3 into a single PR. Each step in isolation breaks the next, so they ship together.
• 2.1: Drops the legacy-allocation BackwardCompatibility e2e shard (legacy allocation is going away).
• 2.2: Floors the surviving (onchain) BackwardCompatibility shard at CLI ≥ 0.12.0 and bumps onchain MIN_COMPATIBLE_VERSION in the serviceability program to match. The 0.14.1 floor for link accept is already covered by the existing before("0.19.0") RFC-18 guard.
• 2.3: Removes the activator container from the e2e devnet entirely; all remaining shards run with onchain allocation. DZ_E2E_ONCHAIN_ALLOCATION and DZ_E2E_DISABLE_ACTIVATOR env knobs are deleted. Shard 3 (the standalone onchain-no-activator shard) is folded into the round-robin shards.

After this PR, the e2e devnet has no path to start an activator — there is no environment variable that can re-enable it. Per the issue's note, breakage in onchain-allocation behavior is no longer masked by the activator picking up the slack; expect the first few CI runs to surface latent issues.

PR size: Exceeds the ~500-line guideline. The three phases were authored as separate issues for that reason but were bundled per request because each step depends on the next.

Closes #3609
Closes #3610
Closes #3611

Testing Verification

• go vet -tags=e2e ./e2e/... clean.
• go build ./... clean.
• cargo test -p doublezero-serviceability and cargo clippy -p doublezero-serviceability --all-targets -- -Dclippy::all -Dwarnings clean after the `MIN_COMPATIBLE_VERSION` bump.
• New unit test `test_create_loopback_with_onchain_allocation_honors_supplied_ip_net` asserts the caller-supplied `ip_net` survives a Loopback create under onchain allocation and that the DeviceTunnelBlock is left untouched.
• `TestE2E_SentinelMulticastPublisherCreatesPublishers` previously failed at the multicast-publisher creation step (`InvalidTunnelEndpoint` → after the contract fix, `AccountAlreadyInitialized` on the second poll). It now passes end-to-end: the sentinel detects the three IBRL validators, creates Activated multicast publisher users on the configured group, and `wait-for-multicast-publishers` completes in ~5s.
• `grep -ri DZ_E2E_ONCHAIN_ALLOCATION` and `grep -ri DZ_E2E_DISABLE_ACTIVATOR` return nothing.
• `grep -ri activator e2e/` returns only the onchain `activator-authority` / `activator_authority_pk` references (program-level authority field, out of scope for phase 2).
• E2E shards in CI: matrix should render as 5 entries — shard 1 (`TestE2E_BackwardCompatibility`) and shards 2–5 round-robin remainder. No `onchain_allocation`/`disable_activator` keys present.

[juan-malbeclabs]

LGTM