Sync monorepo to latest Midnight + Compact compiler versions

[0xisk]

PR #86 moved packages/deployer to the new Midnight stack (midnight-js 4.1.x, ledger-v8, compact-runtime 0.16). The rest of the monorepo never moved with it, and the Compact compiler version is inconsistent across the repo (CI pins 0.28.0, README badge says 0.29.0, local toolchain is 0.31.0).

Bring everything up to the latest published versions.

Simulator (packages/simulator)

• @midnight-ntwrk/compact-runtime 0.14.0 → 0.16.0
• @midnight-ntwrk/ledger-v7 ^7.0.0 → @midnight-ntwrk/ledger-v8 8.1.0

The v7→v8 API is signature-identical for every symbol the simulator uses (QueryContext, CostModel, ChargedState, StateValue, ContractState, encodeContractAddress, …), so the only source change is one fixture import path. Fixtures recompile at test time, so they pick up the new runtime automatically.

Repo-level

• CI compiler pin 0.28.0 → 0.31.0 (.github/actions/setup/action.yml)
• README compiler badge 0.29.0 → 0.31.0

Follow-up (separate, in #86)

• Deployer: midnight-js 4.1.0 → 4.1.1, ledger-v8 8.0.3 → 8.1.0, plus root resolutions refresh. These live only on the feat: compact-deployer tool #86 branch, not on main.