feat(state-store): default evm-ss-split=true for new nodes

[bdchatham]

Adds StateStoreConfig.EVMSSSplit and defaults it true, so newly generated node configs render app.toml [state-store] evm-ss-split = true — the giga SS-store layout. ss-enable/sc-enable already default true, so this is the single extra flag distinguishing a giga SS node from a plain one (per protocol guidance; decoupled from the Giga executor, giga_executor.*, which is untouched).

Changes

• config.go: new EVMSSSplit field on StateStoreConfig (unified tag evm_ss_split).
• legacy.go: renders the bare evm-ss-split key under [state-store] (siblings are ss--prefixed; the binary reads this one unprefixed), mapped both directions for round-trip fidelity.
• defaults.go: EVMSSSplit: true in the baseline.
• enrichments.go: field description.
• io_write_test.go: TestWriteConfigToDir_EVMSSSplit guards default-on render + the bare-key form + omit-when-false.

Safety / blast radius

• Additive optional field, no schema-version bump. Existing v2 configs load the zero value (false) and, via ,omitempty, render no evm-ss-split key — their app.toml is byte-unchanged. Only newly generated configs carry it true.
• Existing nodes are not migrated. Flipping the split on over an unsplit data dir fails seid's startup safety check, so an existing node adopts it only via a state-sync (the controller migration workflow) — unchanged by this PR.

Review focus (the one integration risk)

Does any consumer regenerate a node's config from DefaultForMode on reconcile (rather than load-and-preserve)? If so, existing nodes could pick up evm-ss-split=true without a state-sync and fail to boot. I believe the render path preserves on-disk values (hence omitempty), but flagging for the platform/controller experts to confirm before merge.

🤖 Generated with Claude Code

[cursor]

PR Summary

Medium Risk
Default-on for new full nodes changes storage layout expectations at bootstrap; existing configs are preserved via omitempty, but any path that rewrites config from DefaultForMode without preserving on-disk values could enable split on unsplit data and fail startup.

Overview
Introduces storage.state_store.evm_ss_split and wires it through unified config, legacy app.toml [state-store] as the bare evm-ss-split key (not ss- prefixed), enrichments, and round-trip mapping.

Mode defaults: full nodes get EVMSSSplit = true so newly written configs emit evm-ss-split = true (giga SS layout with EVM state in its own state-store DB). Archive explicitly resets it to false; validator and seed omit the key because state store is disabled. false uses omitempty, so existing on-disk configs without the key stay unchanged on re-render.

Validation rejects evm_ss_split when state_store.enable is false. TestWriteConfigToDir_EVMSSSplit locks the per-mode render behavior and bare-key form.

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