Solver should use indexer subscription to watch the mempool

[louisinger]

1. Instead of GetTransactionStream, the solver should use indexer GetSubscription/UpdateSubscription.

2. CEL filters have been implemented (not released) : https://github.com/arkade-os/arkd/blob/c4f163241451eb05a873a5e61043ff4c16a0b7b8/internal/interface/grpc/handlers/txfilter/README.md
   each plugin must define its own filter (Plugin.Filter() already there but not implemented).

[arkanaai]

Codebase Context

Current implementation — pkg/solver/arkdsource/arkdsource.go

The solver subscribes to mempool transactions via GetTransactionsStream on the arkd Client interface:

solver.go:78  → source.Subscribe(ctx, p.Filter())
arkdsource.go:44 → s.c.GetTransactionsStream(ctx)

This returns an unfiltered stream of all transactions. Each plugin's CEL filter string (set via builder.WithFilter(cel)) is stored but never sent to arkd — filtering happens client-side after receiving the full stream.

What the issue asks for — switch to the arkd Indexer interface instead:

• GetSubscription / UpdateSubscription live in the indexer client (arkd/pkg/client-lib/indexer.go)
• CEL filter support is implemented in arkd/internal/interface/grpc/handlers/txfilter/ (env.go, filter.go, tx.go) but not yet released

CEL environment exposes:

• tx.extension — map of ARK packet types to hex payloads
• hasPacket(extension, packetType) — custom function

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Acceptance Criteria

• arkdsource.Subscribe() calls the indexer's GetSubscription (with CEL filter) instead of GetTransactionsStream
• When a plugin provides a CEL filter string (plugin.Filter() != ""), it is passed server-side at subscription time
• UpdateSubscription is wired so filters can be updated without reconnecting (e.g. if plugin set changes at runtime)
• If arkd returns "CEL not supported" (older server), solver gracefully falls back to unfiltered stream + client-side filtering
• Existing plugin filter expressions (builder.WithFilter(...)) work unchanged against the new server-side evaluator
• Integration test: solver with a CEL filter receives only matching transactions

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Suggested Approach

1. Extend Source interface (pkg/solver/source.go) — no interface change needed if arkdsource just swaps internals.

2. Replace in arkdsource.go:

   • On Subscribe(ctx, filter): call indexerClient.NewSubscription(ctx, filter) → returns subscriptionID + event channel
   • Map ScriptEvent → *psbt.Packet (check if event shape differs from current TransactionEvent)
   • On plugin hot-reload or filter change: call UpdateSubscription(ctx, subscriptionID, add, remove)

3. Thread the indexer client — arkdsource.New(...) currently takes a client.Client; it needs an indexer.Indexer (or a combined client that implements both). Check runtime.go:138 for the construction site.

4. Coordinate with arkd release — the CEL txfilter package is on arkd@main but not in a tagged release yet. Pin the solver to the right arkd module version once tagged.

Key files to touch:

• pkg/solver/arkdsource/arkdsource.go — main change
• internal/solverd/runtime.go:138 — wire indexer client
• go.mod — bump arkd dependency once CEL release is out