circular: add circular rebalancing plugin with bLIP-18 support

[btweenthebars]

Summary

This PR adds circular, a high-performance Core Lightning plugin written in Go for routing nodes to rebalance channels efficiently and cost-effectively.

Key Features:

• bLIP-18 Inbound Fee Aware: Dynamically parses gossip TLV type 55555 for inbound fees, computing exact net fees (max(0, outbound + inbound)) via edge-keyed backwards Dijkstra to discover routes with negative fee discounts.
• Failure-Informed Pathfinding: Automatically excludes failing channels across retry attempts so the pathfinder avoids recurring bottlenecks.
• Parallel Multi-Channel Rebalancing: Simultaneously pull into or push out of target channels across multiple peers (circular-pull and circular-push) with fee-aware candidate selection.
• In-Flight Payment Tracking: Real-time visibility into active parallel rebalancing operations via circular-active.
• Cryptographically Secure & Concurrency Hardened: Uses crypto/rand for preimage generation, atomic state flags, non-blocking channels, and panic-resilient background routines.

Repository: https://github.com/btweenthebars/CLN-circular

[daywalker90]

You can ignore the failing circular-reckless test, but the normal circular test is also failing. You can check the logs here. Apparently circular exits because there is no gossip/channels ?

Btw. we will replace reckless probably in v26.12 and people using that plugin manager to install your plugin will have to call it "CLN-circular" as that is your repos name.

[btweenthebars]

Thanks for the pointer @daywalker90!

The root cause was in glightning.ListChannels(): when there are 0 channels in gossip (as on a clean regtest test node), glightning returns "No channel found for short channel id ", which was previously treated as a fatal startup error by Init.

Fixed in the latest commit:

1. Handled "No channel found" as an empty channel slice rather than an error.
2. Made initial refreshGraph and refreshPeers in Init() non-fatal so the plugin starts and registers cleanly even before gossip has arrived.
3. Enhanced the integration test in tests/test_circular.py to verify circular-stats RPC execution.

Submodule pointer updated to 2d2d1a6.

[daywalker90]

I fixed up the README and Reckless markdown files (alphabetical ordering and badge links) and squashed the commmits.