fix(core)(#191): promote errMessage helper; fix lossy NametagMinter stringification

[vrogojin]

Summary

Fixes #191 — Sphere.init({ nametag }) failures surfacing as Submit failed: [object Object] instead of the aggregator's structured rejection.

• Promote the existing errMessage helper from modules/payments/transfer/nostr-persistence-verifier.ts to core/errors.ts, next to redactCause / SphereError. The helper routes unknown-shape errors through redactCause + JSON.stringify, preserving field-level forensics while keeping W40 secret-byte redaction intact.
• Replace the two highest-visibility lossy callsites in modules/payments/NametagMinter.ts (submit-retry catch at :182 and the outer mintNametag catch at :256) with errMessage(error).
• Update nostr-persistence-verifier.ts to import the promoted helper instead of defining its own copy.
• Add tests/unit/core/errors.errMessage.test.ts covering: Error instances, custom Error subclasses, string throws, structured-object throws (the issue Lossy error stringification in NametagMinter — [object Object] masks aggregator failure reason #191 reproducer), aggregator-shaped payloads, W40 redaction integration, arrays, null/undefined, BigInt unstringifiables, and cycle-safety.

Out of scope

The remaining ~89 occurrences of err instanceof Error ? err.message : String(err) in core/ and modules/ are deferred per the issue's "Optional follow-up" note — they surface in warn-logs / internal worker paths rather than user-facing errors. A bulk replacement would inflate this PR's scope.

Test plan

• npx vitest run tests/unit/core/errors.errMessage.test.ts — 10/10 pass
• npx vitest run tests/unit/payments/transfer/sphere-error-redaction.test.ts — 44/44 pass (no W40 regression)
• npx vitest run tests/unit/modules/NametagMinter.test.ts tests/unit/payments/transfer/nostr-persistence-verifier.test.ts — 46/46 pass
• npx vitest run tests/unit/core/ — 451/451 pass
• npx vitest run tests/unit/payments/ tests/unit/modules/ — 3181/3181 pass
• npm run typecheck — clean
• npx eslint on the four changed files — clean
• Manual verification against goggregator-test that an actual nametag-mint rejection surfaces a JSON payload (not [object Object]) — requires testnet access

Before / after

Before:

SphereError: Failed to mint nametag token: Submit failed: [object Object]

After (when aggregator returns a structured error):

SphereError: Failed to mint nametag token: Submit failed: {"status":"BAD_REQUEST","reason":"..."}