Add initial Nebula Core packages and docs

Import initial monorepo structure for Nebula Core: add packages (@nebula/core, core-glyphs, core-input, core-navigation, core-theme, core-ui, core-utils) with source, dist, tests and assets. Expand README with overview, quick start and API snippets, and add package-level documentation files. Add jsconfig.json for path mapping, package.json and lockfiles to bootstrap the repo. This commit sets up the project layout, docs, and local package links for further development.

packages/core-glyphs/README.md

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+# @nebula/core-glyphs
+
+Controller glyph mappings and lookup helpers. Data-only, no rendering logic.
+
+## Why it exists
+Controller prompts should be consistent across Steam Deck, Xbox, and PlayStation. This package keeps glyph mapping in one place.
+
+## Usage
+
+```js
+import { getGlyph, getGlyphAssetPath } from "@nebula/core-glyphs";
+
+const glyph = getGlyph("steam-deck", "confirm");
+console.log(glyph); // "A"
+
+const asset = getGlyphAssetPath("steam-deck", "confirm");
+console.log(asset); // "assets/Steam Deck/SteamDeck_A.png"
+```
+
+## SteamOS / Steam Deck notes
+Provide Deck-specific prompts by selecting `steam-deck` when the device is detected. Use the asset path helper or bind your own icon assets keyed by the glyph labels.