feat(contribs): add gnoupgrade chain upgrade testing toolkit

[moul]

Summary

Adds contribs/gnoupgrade — a CLI toolkit for testing and verifying gno.land chain upgrades before they touch real chains.

Subcommands

• replay — Starts a gnoland node against an existing data directory to verify the new binary can replay all committed blocks without panic. This is the primary smoke test gate for chain upgrades.
• statediff — Captures chain state (chain ID, height, app hash, realm/package listing) in a canonical, sorted JSON format. Run against old and new binaries and diff the output to see exactly what changed.
• healthcheck — Runs post-upgrade health checks against a running node: connectivity, block production, ABCI query.

Context

This is part of the chain upgrade confidence-building pipeline (braindump). It implements Tools 1-3 from the plan:

1. Replay smoke test (does the new binary crash on existing state?)
2. Integration health checks (does the chain actually work after upgrade?)
3. State diff / snapshot comparator (what exactly changed?)

Usage

# Smoke test: replay chain data with new binary
gnoupgrade replay --data-dir /path/to/gnoland-data --timeout 30m

# Capture state snapshot for comparison
gnoupgrade statediff --remote http://node:26657 --output before.json
# ... upgrade ...
gnoupgrade statediff --remote http://node:26657 --output after.json
diff before.json after.json

# Post-upgrade health check
gnoupgrade healthcheck --remote http://node:26657

Related

• feat(node): add --halt-height flag for coordinated chain upgrades #5334 — --halt-height CLI flag (Phase 1)
• feat(node): add GovDAO-based chain halt via r/sys/params #5368 — GovDAO-based halt height (Phase 2)

Test plan

• go build ./... passes
• gnoupgrade --help shows all three subcommands
• gnoupgrade replay --help shows all flags
• Manual test: replay against a test12 data snapshot
• Manual test: statediff against a running node
• Manual test: healthcheck against a running node

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