Detect Ricochet at any hop (0-4) by tracing backward to fee address

[4rkad]

Background

Issue #36 describes Ricochet detection via the known fee address at hop 0. This issue proposes extending detection to identify Ricochet transactions at any hop (1-4) by walking backward through the chain until the hop 0 fingerprint is found.

Currently, detectRicochet() in spending-patterns.ts only detects generic 1-in-1-out sweep chains and can only look 1 hop back (line 102: "Can't walk further without grandparent data"). It does not check for the known fee address at all.

Proposed algorithm

When analyzing a transaction that is 1-input, 1-output (sweep pattern):

1. Walk backward up to 4 hops via traceBackward() (already exists in recursive-trace.ts)
2. At each ancestor, check two things:
   • Is it also a 1-in-1-out sweep? (continue walking)
   • Does it have an output to the known fee address with the exact fee amount? (hop 0 found)
3. If hop 0 is found, label the analyzed transaction with its hop number (distance from hop 0)
4. Validation: confirm amounts decrease by ~960 sats (miner fee) per hop for extra confidence
5. Early abort: if an ancestor is not 1-in-1-out and doesn't have the fee address output, stop - it's not a Ricochet chain

Known fee addresses and amounts

Wallet	Fee address	Fee amount
Ashigaru	bc1qsc887pxce0r3qed50e8he49a3amenemgptakg2	100,000 sats
Samourai	(to be confirmed - no longer active)	200,000 sats

Implementation notes

• Modify detectRicochet() in src/lib/analysis/chain/spending-patterns.ts
• Use traceBackward() from src/lib/analysis/chain/recursive-trace.ts with maxDepth: 4
• The finding should include: which hop the analyzed tx is, the full chain of txids, and whether it's classic (consecutive blocks) or staggered (spaced blocks)
• API cost: max 4 extra calls per candidate tx, only triggered for 1-in-1-out transactions
• PayNym variant remains undetectable by design - document this in the analysis output when a Ricochet is detected (recommend user to use staggered + PayNym)

Scope

This is a sub-task of #36. The parent issue covers full Ricochet detection (hop 0 via fee address + chain tracing forward). This issue specifically covers the backward detection case: identifying a Ricochet when the user analyzes a transaction that is hop 1, 2, 3, or 4.

Example

Using the chain from #36:

If a user analyzes tx db837f91... (hop 2), the tool should:

1. See it's 1-in-1-out (candidate)
2. Walk back to cb7bcde5... (hop 1) - also 1-in-1-out, continue
3. Walk back to 038a94db... (hop 0) - has output of 100,000 sats to bc1qsc887pxce0r3qed50e8he49a3amenemgptakg2
4. Confirm: "This transaction is hop 2 of a Ricochet (Ashigaru)" with link to full chain