docs: add CHANGELOG.md (Keep a Changelog format)

CHANGELOG.md

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+# Changelog — nu-proto
+
+All notable changes to this project will be documented in this file.
+Format: [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)
+
+---
+
+## [Unreleased]
+
+## [0.1.0] — 2026-04-24
+
+### Added
+- `types/transaction.proto` — `Transaction`, `TxPayload` oneof with all variants:
+  `TokenTransfer`, `NodeSubmit`, `VoteRegister`, `VoteCast`, `NftTransfer`,
+  `CollectionClaim`, `StakeOp`, `ValidatorRegister`; auto variants:
+  `NodeApprove`, `NftMint`, `NodeReject`, `VotingOpen`
+- `types/block.proto` — `BlockHeader`, `Block`, `TxReceipt`
+- `types/story_node.proto` — `StoryNodeState`, `NodeStatus` enum, `WeightedVote`
+- `types/nft.proto` — `NftState`, `CollectionState`
+- `rpc/api.proto` — JSON-RPC v0.1 contract: `nu_getBlock`, `nu_getTx`,
+  `nu_getAccount`, `nu_getStory`, `nu_getNode`, `nu_getNft`, `nu_listStories`,
+  `nu_listPendingVotes`, `nu_sendRawTx`, `nu_chainInfo`
+- WebSocket `SubscribeRequest` / `SubscribeEvent` for real-time block and vote updates