feat: support multisig assets and output listing

[cw35]

Summary

• support Safe multisig balances through safe assets --receivers --threshold
• support legacy multisig balances without the retired multisig asset endpoint
• add output list for Safe and legacy outputs with state, asset, offset, limit, and order filters
• resolve legacy kernel asset IDs and guard legacy pagination against duplicate or stalled cursors
• use ascending order by default and emulate descending order client-side where the API is unreliable

Test Coverage

Multisig assets
├── Safe group validation and multi-page unspent aggregation: tested
├── Legacy group validation and multi-page balance aggregation: tested
├── Duplicate output suppression and stalled cursor detection: tested
├── Safe asset metadata batch lookup, fallback, and error propagation: tested

Output list
├── Safe DESC pagination and exclusive offset: tested
├── Legacy ASC/DESC pagination and asset filtering: tested
├── Kernel asset ID resolution: tested
├── Stalled cursor detection: tested
└── Empty result serialization as []: tested

Tests: 0 → 3 test files (+3 new)

Pre-Landing Review

• Initial review found four issues: duplicate legacy pagination, swallowed metadata errors, missing legacy kernel asset resolution, and expensive default DESC behavior.
• All four were fixed and regression-tested.
• Final review found no blocking issues.

Design Review

No frontend files changed — design review skipped.

Eval Results

No prompt-related files changed — evals skipped.

Test plan

• go test ./...
• go test -race -vet=off ./...
• go vet ./cmd/asset/list ./cmd/output ./cmd/safe
• go build ./...
• real API check for /safe/assets/fetch
• real API check that legacy kernel asset filtering returns a JSON array

Documentation

• README.md: documented Safe and legacy multisig balance commands.
• README.md: documented output state/asset filters, ASC default, DESC scan behavior, and offset formats.
• Other documentation files: none present.