Rename packages/6502 directory to packages/core + document the published package

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What

Follow-up to PR #153 (which renamed the npm package @learn6502/6502 → @learn6502/core): this renames the on-disk workspace directory packages/6502 → packages/core so the folder matches the package name, and rewrites the package's README now that it is published to npm.

Directory rename

• git mv packages/6502 → packages/core (the repository.directory field now points at packages/core).
• Only build-critical path reference fixed: packages/translations/build.js xgettext glob ../6502/src → ../core/src. (No meson/gradle/tsconfig/CI/Flatpak path references exist — the workspace resolves members via the packages/* glob and by package name.)
• Renamed .cursor/rules/package-6502.mdc → package-core.mdc and updated its globs:.
• Updated doc references (AGENTS.md, root README.md) and the GitHub directory URLs (tree/main/packages/6502 → …/packages/core) in tutorial/web content, i18n JSON, .po/.pot and Android strings.xml — those links would 404 after the rename. URLs are changed in both msgid and msgstr so gettext keys stay consistent (no orphaned translations); translation files are otherwise untouched (no regeneration churn).
• Deliberately left untouched: external links (6502.org, obelisk.me.uk/6502), the app id eu.jumplink.Learn6502, the @learn6502 scope, and CPU/opcode prose. A blanket 6502/→core/ replace would have corrupted these — only the unambiguous packages/6502 string and the single ../6502/ relative glob were changed.

README

Rewrote packages/core/README.md to document @learn6502/core as a standalone, dependency-free, cross-runtime (Node/GJS/browser) library: what it does, install (npm/yarn/pnpm), a quick-start, the four core classes, the typed event model, disassembly, the memory map, utilities and provenance. All API names, signatures and event names were verified against the package source.

Verification (local)

• gjsify run check (full workspace type-check) — ✅ exit 0
• gjsify workspace @learn6502/core build — ✅
• translations build under Node (exercises the ../core/src glob + po2json) — ✅ exit 0
• exhaustive grep: no remaining packages/6502 or ../6502/ path references; external 6502 links/prose intact.

Discovery was done with an 8-agent sweep (build/CI/source/content/pipeline/bare-token) + adversarial false-positive audit to make sure no directory reference was missed and no 6502 false positive was changed.