Introduce a common glossary file

GLOSSARY.md

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+# Glossary
+
+Short, strict definitions of terms used in the technical docs: common
+terminology across projects. A new definition must not introduce ambiguity
+into the glossary: conflicting terminology must be resolved. An entry links
+at most one external reference — the most authoritative source to research
+further. Definitions should include the exact names a term goes by in other
+places — code identifiers, crate or method names.
+
+Entries are alphabetical.
+
+---
+
+**Bulletin** — The Polkadot Bulletin chain
+([polkadot-bulletin-chain](https://github.com/paritytech/polkadot-bulletin-chain)):
+a *system chain* that provides storage. Calls are feeless; instead they are
+gated by an authorization model —
+an account is authorized in advance to upload, and authorizations expire.
+Data is uploaded by submitting extrinsics; the uploaded data itself is kept
+in off-chain storage, not in the runtime state, and has a retention period.
+
+**Fellowship runtimes** — The production runtimes of Polkadot, Kusama and
+their *system chains*, maintained by the Polkadot Technical Fellowship in
+[polkadot-fellows/runtimes](https://github.com/polkadot-fellows/runtimes) and
+enacted via on-chain referenda. The runtimes developed there are based on a
+specific *SDK release* and released on their own cycle.
+
+**HOP** — Hand-Off Protocol: a JSON-RPC protocol (`sc-hop`) exposed by
+*Bulletin* collators. Bulletin-authorized accounts submit data that resides
+on that one collator node only, addressed to one or more recipients (one-time
+public keys) who claim it. Data not claimed within a configurable retention
+period is promoted best-effort: submitted to *Bulletin* as an extrinsic.
+
+**Node** — The binary running a chain: networking, database, consensus, RPC.
+Runtime state is opaque data to it; a chain spec defines what it runs —
+genesis state, including the runtime, chain id and bootnodes. It exposes
+JSON-RPC on one port for external clients and the P2P protocols on another
+for other nodes. It can run in different modes — collator/validator, bootnode,
+or a regular RPC node; whether it prunes block history and runtime state, or
+keeps all of it (archive node), is an orthogonal choice.
+
+**Pallet** — A FRAME module implementing one domain of runtime logic — its
+storage, calls, events and errors. A runtime is composed of pallets
+(`pallet-balances`, `pallet-staking`, …).
+
+**Runtime API** — A versioned interface (`sp-api`) the Wasm runtime exposes
+to the node, invoked on a specific block's state (block building, transaction
+validation, metadata, …). Not the node's JSON-RPC: RPC is served by the node
+to external clients, a runtime API is called by the node into the runtime.
+
+**SDK release** — A versioned release (`stableYYMM`) of
+[polkadot-sdk](https://github.com/paritytech/polkadot-sdk), the crate set that
+node and runtime implementations build against. Ships on a regular cadence,
+independent of *Fellowship runtimes* releases.
+
+**System chain** — A parachain that is part of the network's protocol itself.
+Runs on behalf of the network; its runtime lives in the *Fellowship
+runtimes*. Uses a para ID below 2000 — the range that is not publicly
+registrable (`LOWEST_PUBLIC_ID` in polkadot-sdk).