Register Seneschal builder

[Rotwang9000]

Summary

Adds an entry for Seneschal to builder-registrations.json.

• Builder name: Seneschal
• RPC: https://rpc.seneschal.space
• Supported APIs: v0.1, refund-recipient, cancel-endpoint
• On-chain extra_data: Seneschal/0.1
• Operator contact: @Rotwang9000 on Discord
• Stack: rbuilder fork on a single co-located node (32 cores) in Helsinki.
  Currently bidding into Aestus, Agnostic, ETHGas, Flashbots, Titan, and
  Ultrasound mainnet relays. Block share is below the 2 % criterion right
  now; this PR is to signal intent and compatibility, per the README:

  The purpose of this registry is to signal intent, compatibility,
  and agreement with the principles.

Fair Market Principles attestation

The Seneschal operator commits to the Fair Market Principles in
fair-market-principles.md. Specifically:

• Bundles received from MEV-share matchmakers will not be sold or shared
  with other parties (no other builders, no internal or external
  searchers).
• Bundle atomicity will be preserved: either all transactions land in
  the intended order or none do. The only exception is opt-in
  conflict-free merging where the user has consented.
• Bundle privacy (including failed-trade privacy) will be respected;
  no sensitive bundle data will be shared outside the builder.
• Validity conditions will be correctly implemented, including correct
  refund transfers to the configured recipients (the upstream rbuilder
  implementation in crates/rbuilder/src/building/mod.rs is used
  unmodified for this).
• We will not take actions that negatively impact bundle execution
  quality (price or refund amount).
• We will publicly document the bundle-merging algorithm and any
  notable features once the public ops page is live; for now, the
  default rbuilder algorithms mp-ordering, mgp-ordering and
  parallel-builder are in use without modification.

Reachability

$ curl -X POST -H 'content-type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"eth_sendBundle","params":[]}' \
  https://rpc.seneschal.space/
{"jsonrpc":"2.0","result":null,"id":1}

$ curl https://rpc.seneschal.space/health
ok

TLS via Let's Encrypt. Body size capped at 1 MB. Only POST accepted;
GET returns 405.