new env var SOLVER_EXPLORER_URL

[bordalix]

allows passing explorer url as env var for arksdk Init

closes #8

[arkanaai]

Review: #9 — SOLVER_EXPLORER_URL env var

Verdict: APPROVE ✅

Clean, minimal change. Adds an optional SOLVER_EXPLORER_URL env var that gets passed through to arksdk.WithExplorerURL() during wallet init. Reviewed all 5 changed files against the go-sdk InitOption contract.

What's good

• Correctly optional: ExplorerURL defaults to empty string, and service.go:209-211 only appends the WithExplorerURL option when non-empty. This avoids triggering the go-sdk's "explorer url cannot be empty" validation error (go-sdk/init_opts.go:23).
• Proper go-sdk integration: WithExplorerURL and InitOption are confirmed exported in go-sdk at the pinned version (v0.9.2-0.20260518112312-588477f9d618). The variadic Init(...opts) signature matches.
• Tests cover both paths: config_test.go asserts ExplorerURL is empty by default and correctly overridden via env var. Good.
• Config layer is clean: follows the existing viper pattern exactly — add var, add struct field, read via viper.GetString().
• Not protocol-critical: this only affects which block explorer the go-sdk wallet talks to for chain queries. No VTXO, signing, forfeit, or round lifecycle code is touched.

One bug in docker-compose

test/docker-compose.yml:70 — SOLVER_EXPLORER_URL=http://chopsticks:3000 is set on the arkd container, not the solver. The arkd image doesn't read SOLVER_* env vars — this env var is dead weight here. It should be set on whatever runs the solver process during tests (or in a solver service block if one gets added to this compose file). Not a blocker since tests likely set this env var separately, but worth fixing so the compose file isn't misleading.

No cross-repo breakage

• No public API, proto, or type changes — this is purely internal config plumbing.
• go-sdk already exports WithExplorerURL; no SDK changes needed.
• README table updated correctly.

LGTM. Ship it, fix the docker-compose nit when convenient.

[louisinger]

included into 3b934ac