refactor(js): flatten and reorganize js/src TS files (#54)

[schloerke]

Summary

Behavior-preserving refactor of the IIFE entry module so every top-level file under js/src/ has one clear responsibility and index.ts reads as a boot sequence rather than a kitchen sink. Closes #54.

• Extract output-binding.ts — moves ShinyreactOutputBinding out of index.ts behind an explicit registerShinyreactOutputBinding().
• Extract global.ts — moves the Window.shinyreact declaration + window.shinyreact = Object.assign(...) assignment behind an explicit installGlobal().
• Rename spec.ts → types.ts — the file is exclusively wire-tree TS types; exported type names unchanged, 5 importers repointed.
• index.ts is now ~9 lines: CSS import + installGlobal() → registerShinyreactOutputBinding() → installInlineSpecSeeding().

No import-time side effects for our own modules (only the unavoidable ./shinyreact.css). No change to runtime behavior, exported type names, or the window.shinyreact surface. Vendored js/src/shiny-react/ untouched (its fate is downstream of #28).

Of the issue's five proposals: #3 (test colocation) was already done; #5 (vendored dir) is out of scope; this PR does #1, #2, and #4.

Test Plan

• make js-lint (tsc --noEmit) clean
• npx vitest run — 183 tests pass
• make js-build produces a byte-similar dist/shinyreact.js (diff is bundler-internal mangled-symbol renames only; +40 bytes from one added doc comment)
• make update-dist refreshed the bundled copies in pkg-py/ and pkg-r/

Spec and plan: docs/superpowers/specs/2026-06-10-flatten-js-src-design.md, docs/superpowers/plans/2026-06-10-flatten-js-src.md