feat: add read-only multiroom inspection

[zzwong]

Summary

• add read-only group status and group members commands
• normalize modern Arylic and legacy Linkplay multiroom response shapes
• derive master/slave/standalone/unknown roles with stable human and JSON output
• harden device JSON decoding against precision loss, duplicate keys, excessive nesting, malformed fields, oversized groups, and terminal injection
• document published API provenance, safety boundaries, and compatibility limits

Examples

wiim group status
wiim --json group members
wiim --device living-room group status

Safety

No first-class join, leave, kick, volume, mute, or channel mutations are included. Both commands issue read-only status/member queries only.

Hardware verification

Verified read-only against a standalone WiiM Ultra: WMRM 4.3 and zero-member response rendered correctly. Master/slave grouped configurations remain explicitly documented as not yet hardware-verified.

Verification

• gofmt -l . clean
• go vet ./...
• go test -race ./...
• golangci-lint run ./... (0 issues)
• Linux/386 parser tests pass
• live standalone group status and group members succeeded

Reviews

• task-level spec and quality gates passed
• final full-diff spec review passed and quality review approved
• adversarial findings addressed: case-fold ambiguity, numeric precision/bounds, list allocation limits, terminal escaping, exact duplicate JSON keys, and nesting-depth denial of service
• final adversarial re-review: CLEAN