feedback5 R6: add a small checkJs slice and core typedefs

tsc checks shared (except protocol.js and bandBonds.js), the sim rng/dir/constants helpers, and types/core.js. strict stays off. types/README.md says how to widen the slice.

types/README.md

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+# Typecheck
+
+`npm run typecheck` runs `tsc --noEmit --checkJs -p jsconfig.json`.
+
+The first slice is the closed set that already typechecks:
+
+- `shared/` except `shared/protocol.js` and `shared/bandBonds.js` (their JSDoc does not match the code yet; see below)
+- `server/sim/rng.js`, `server/sim/dir.js`, `server/sim/constants.js` (the last imports `GEO` from shared)
+- `types/` — JSDoc for `Unit`, `DamageInfo`, the snapshot tuples, and the protocol message envelope
+
+`strict` is off. `tsc` does not read `jsconfig.json` on its own; the script passes `-p`.
+
+## Widen it
+
+Add one file or one folder to `jsconfig.json` `include`, then run `npm run typecheck`. Fix JSDoc that disagrees with the code in that folder. Leave `strict` off until the folder reports nothing. Do not add `@ts-nocheck`.
+
+Suggested order once the sim split settles: `server/sim/grid.js`, `server/sim/body.js`, `server/sim/buffs.js`, then `server/sim/damage.js` against the typedefs in `types/core.js`. The sim does not import `types/`. The typedefs are a reference, not a runtime dependency.
+
+`types/core.js` follows docs/DESIGN.md §5.2, §5.4, §8 and docs/SIM.md §4, §9. Where those two disagree on `DamageInfo.type`, the typedef uses the union `makeDamageInfo` actually builds (`'elemental'` as well as `'element'`).
+
+## Not in the first slice
+
+`shared/protocol.js` — the catalogue JSDoc is ahead of a few object literals (`$optional` sits on a `Record` of checkers; `unitStatsEntry` does not list `side` or `silenced`, and the code reads `side`). Fix the comments, then remove it from `exclude`.
+
+`shared/bandBonds.js` — the data-shape JSDoc rejects an empty fallback object and a tuple callback. Same treatment.