feat: add read-state upgrade handler

[0xNilesh]

Upgrade handler for the x/ucallback read-state module that landed in #310.

What it does

• Adds the ucallback store. New module, so StoreUpgrades.Added is required — RunMigrations registers the consensus version but cannot create a store the multistore was never told about, and the node fails to load at the upgrade height without it.
• Reserves every unclaimed system-contract address. SYSTEM_CONTRACTS is 47 entries, not 7 — constants.go's init() fills the 0xA0-0xAF / 0xB0-0xBF / 0xC0-0xCF ranges with a full proxy+admin+impl triple each. The genesis loop only runs at InitGenesis, so slots added after a chain launched were never reserved on it. Checked against live donut: 41 of the 47 are bare, including UNIVERSAL_CALLBACK (0x…C2), which x/ucallback needs before it can ingest anything. The other 6 are skipped by the already-deployed guard.

Two fixes it depends on

Both are chain halts found by running the upgrade locally, not by tests.

5f1607ab — EVM coin-info fallback. x/vm sets the coin-info global in its own PreBlock, but x/upgrade's PreBlocker runs first. Any handler touching EVM state resolves the coin denom through a still-nil global and the node dies mid-upgrade. Upstream provides a fallback for exactly this window; nothing here was populating it. Split by build tag — under -tags=test that setter writes the coin info rather than a fallback and collides with vm.InitGenesis.

4af77abc — code-hash check. isContractDeployed went through GetAccount, which loads the balance and hits the same global. Now reads the code-hash store directly. Same semantics, same F-2026-17025 sentinel guard.

FixReservedBytecode survived on donut only because it writes code without ever reading an account.

Verification

Local cosmovisor simulation, release/v1.1.39-donut → this branch, passed first attempt:

baseline          6 deployed / 41 bare  — mirrors donut
halt @97          old binary stops, cosmovisor swaps
handler           ran clean, 0 panics, 1 attempt
resumed           97 -> 174, catching_up: false
upgrade applied   height 97
ucallback         registered v1
contracts         47 checked, 0 missing

To reproduce donut's state the OLD binary was built with auto-reservation disabled — the release branch already reserves all 47 at genesis, so a stock fresh chain cannot reproduce the gap.

Plan name must be exactly read-state.