fix(sn-manager): replace EVM rollback stack with minimal upgrade gate

[mateeullahmalik]

Summary

Replace the broad sn-manager EVM preflight/rollback implementation with the minimal forward-only gate already merged and released through #315.

Commit sequence is intentionally auditable:

1. revert fix(sn-manager): remove automatic EVM rollback #313;
2. revert sn-manager: EVM preflight + auto-rollback (v2.6.1-testnet fix) #311;
3. cherry-pick fix(sn-manager): gate unprepared v2.6 upgrades #315's minimal gate onto master.

The resulting tree is exactly master immediately before #311 plus the four-file #315 patch.

Behavior

Current SuperNode	Target	evm_key_name	Automatic result
<2.6	<2.6	any	unchanged
<2.6	>=2.6	non-empty	allow
<2.6	>=2.6	missing/blank/unreadable	block before download/install
>=2.6	forward target	absent	allow; never auto-rollback

The v2.6.0-testnet prerelease is treated as a v2.6 binary by comparing core versions.

Rationale

evm_key_name is transitional migration input, not durable proof of runtime validity. Its absence must not trigger an automatic downgrade. The manager only needs to prevent an unprepared pre-v2.6 node from crossing the v2.6 boundary.

#311/#313 expanded this into chain probing, rollback/history state, cross-process locking, channel/bootstrap changes, startup key validation, migration write-order changes, and related observability. Those concerns should not remain bundled into this narrow compatibility gate.

Explicit removals

This PR intentionally removes the behavior and hardening introduced by #311/#313, including:

• automatic v2.5 rollback/download/symlink/config/restart mutation;
• chain EVM probe and sticky preflight markers;
• rollback/status marker reporting;
• updater and config cross-process locking;
• active-symlink monotonicity hardening;
• init/get/check/start channel/bootstrap changes;
• SuperNode startup active-key algorithm validation added in fix(sn-manager): remove automatic EVM rollback #313;
• migration config-write/delete ordering added in fix(sn-manager): remove automatic EVM rollback #313;
• fix(sn-manager): remove automatic EVM rollback #313 migration-document additions;
• the dedicated nested sn-manager-tests workflow job added in fix(sn-manager): remove automatic EVM rollback #313.

The last item is useful CI coverage but is intentionally not retained inside this behavior-replacement PR. It can be proposed separately without coupling it to updater semantics.

Risks

• Updater hardening unrelated to the minimal gate is removed with the exact reverts.
• An already-v2.6 node with invalid runtime key state is no longer handled by manager rollback or fix(sn-manager): remove automatic EVM rollback #313's additional startup algorithm check; existing SuperNode migration/startup behavior remains authoritative.
• Blocked updates are visible in logs but no longer persisted in sticky status-marker state.

Partial failure

On a blocked transition, the updater returns before tarball lookup. The installed binaries, active symlink, persisted manager config, and restart marker remain unchanged.

Rollback strategy

Revert this PR as one unit before a new release. Do not operationally reintroduce automatic v2.5 rollback on nodes whose active keyring has already migrated to EVM format.

Migration

No chain or filesystem migration is required. This removes #311/#313 marker/locking behavior and retains the existing config schema plus the narrow read of supernode.evm_key_name.

Observability

• Emits an explicit log when the automatic v2.6 boundary transition is blocked.
• Removes sticky update-blocked.log and historical rollback status behavior introduced by sn-manager: EVM preflight + auto-rollback (v2.6.1-testnet fix) #311/fix(sn-manager): remove automatic EVM rollback #313.

State-machine impact

None.

• State keys: unchanged
• CheckTx / DeliverTx: unchanged
• BeginBlock / EndBlock: unchanged
• Determinism / replay: unchanged
• Chain migration / upgrade handler: none

Verification

Using repository-required Go 1.26.2:

• go test ./... in sn-manager
• go test -race ./internal/updater ./internal/utils in sn-manager
• go vet ./... in sn-manager
• CGO_ENABLED=0 go build ./... in sn-manager
• make test-unit
• root go vet ./...
• go test -count=1 -v ./tests/integration/...
• go mod tidy in both modules: no diff
• git diff --check
• resulting tree hash matched a mechanical reconstruction of pre-sn-manager: EVM preflight + auto-rollback (v2.6.1-testnet fix) #311 master plus fix(sn-manager): gate unprepared v2.6 upgrades #315
• four fix(sn-manager): gate unprepared v2.6 upgrades #315 behavior files are byte-identical to the branch previously validated with official v2.5.2 and v2.6.1-testnet release binaries