app: distribute cluster proposer config (fee recipient / gas limit) to validator clients

🎯 Problem to be solved

Charon currently ignores the VC's proposer config: prepare_beacon_proposer is a no-op (core/validatorapi/router.go, submitProposalPreparations) and register_validator is ignored (core/validatorapi/validatorapi.go, SubmitValidatorRegistrations). Charon injects the lock's fee recipient itself. As a result the VC-side fee recipient / gas limit are unmanaged and irrelevant today — CDVN VCs set none, and Teku even hardcodes --validators-proposer-default-fee-recipient 0x0000…0000.

Post-gloas the VC signs fee_recipient and target_gas_limit into SignedProposerPreferences (#4691), so its local config becomes load-bearing and must match the lock cluster-wide or the partial signatures won't aggregate. This issue: give Charon a way to push the cluster-canonical fee recipient + gas limit (lock FeeRecipientAddresses / TargetGasLimit) into the VCs.

This has independent, pre-gloas value — it makes VCs hold the correct fee recipient rather than relying on Charon's override — so it can land before the gloas duty.

Sub-task of the gloas epic #4324.

🛠️ Proposed solution

Findings

• No standard config file exists. The ecosystem is standardizing an API instead — keymanager-APIs#87 (/eth/v1/validator/config, closed/contentious, not ratified), explicitly to kill the "different config dialect per client" burden it names for dappnode/eth-docker.
• Per-client config surfaces:
  • Prysm / Teku / Lodestar — one central proposer_config JSON: --proposer-settings-file / --validators-proposer-config / --proposerSettingsFile.
  • Nimbus — per-validator files <validators-dir>/<pubkey>/suggested_fee_recipient.hex + suggested_gas_limit.json.
  • Lighthouse — validator_definitions.yml (keystore-entangled; standalone fee-recipient file removed); CDVN profile authors it from proposer-config.json on every container start (ObolNetwork/charon-distributed-validator-node#487).
• Applying updates (matters for coordinated gas_limit bumps, which are applied staggered across operators): file rewrite + VC restart everywhere. File hot-reload exists only on Teku (--Xvalidators-proposer-config-refresh-enabled, per epoch) and Nimbus (per-validator files read live); Prysm/Lodestar re-read on restart; the CDVN Lighthouse adapter regenerates validator_definitions.yml from proposer-config.json on every container start, so a restart applies updates there too.

Mechanism

File-based, exclusively. The keymanager API is ruled out permanently (decision 2026-09-17) — charon generates proposer-config.json, per-client adapters derive native formats from it, and updates apply via file rewrite + VC restart. Post-gloas this same file is the alignment surface for the values VCs sign (SignedProposerPreferences, and later the builder url/auth_data signed into BuilderRequestAuth), so it must be byte-consistent across the cluster.

Tasks

• Decide mechanism: (B) per-client file generation — one JSON parsed natively by Prysm/Teku/Lodestar (JSON is valid YAML); Nimbus + Lighthouse via generate-from-JSON adapters (LH: author the whole validator_definitions.yml — a supported workflow — generate, not merge). Always-on (no feature flag — the diff is small and generation is best-effort/create-once, decided in review of #4700).
• Resolve canonical config from the lock: keyed by this node's pubshares (the identities the VC manages); fee_recipient from FeeRecipientAddresses, gas_limit from TargetGasLimit with registration/30M fallback (#4700)
• keymanager path ruled out permanently — file-based is the mechanism
• file path: generate proposer-config.json next to the lock, create-if-missing, best-effort (#4700)
• Adapters: CDVN run.sh layer (all 5 VCs) — Prysm/Teku direct flag (native parse); Lodestar flag + extension shim (cp to .yml: its readFile throws UnsupportedFileFormat for .json, but content parses — unknown builder.enabled ignored, gas_limit must be string; verified in packages/cli/src/util/proposerConfig.ts); Nimbus renderer → per-validator suggested_fee_recipient.hex + suggested_gas_limit.json; Lighthouse (new CDVN profile) renderer → full validator_definitions.yml generation (replaces import; --disable-auto-discover). Shared: wait-for-file guard (docker creates a directory when bind-mounting a missing file path), restart-semantics docs (hot: Teku w/ refresh flag, Nimbus; restart: Prysm/Lodestar/Lighthouse)
• CDVN wiring: mount the config file per VC profile; replace Teku's hardcoded zero fee-recipient default (ObolNetwork/charon-distributed-validator-node#486, #487)
• charon edit-in-sync flow: extend charon alpha feerecipient fetch (or sibling vc-config sync) to rewrite proposer-config.json from the effective quorum-gated state — the existing sign → Obol-API-quorum → fetch flow is the sync point; sign should print the follow-through (fetch on every node + VC restart on Prysm/Lodestar/Lighthouse). Impl notes: export/move the unexported generator helpers from app for cmd/ use; rewrite via temp-file+rename (atomic vs Teku's periodic re-read); #4693's mismatch metric detects laggards

🔗 References

• Epic: #4324
• Proposer preferences duty: #4691