refactor: move embedded runtime JS to real .js files (js2c) + enum eval caching

[edusperoni]

What

Moves the ~26KB of JavaScript that was embedded as C++ string literals into real, version-controlled .js files under NativeScript/runtime/js/, compiled into the framework at build time (Node-style js2c), and adds two steady-state perf fixes for jitless mode.

Stacked on #409; draft until that lands.

Extraction & build

• tools/js2c.mjs (Node CLI) converts runtime/js/*.js into a generated RuntimeBuiltins.{h,cpp} table (byte arrays, deterministic output). Supports --minify via esbuild (whitespace+syntax only) — implemented but not enabled in any configuration.
• New "Generate RuntimeBuiltins" shell build phase on the NativeScript target, incremental via input/output xcfilelists (skips when nothing changed; verified). Output dir NativeScript/runtime/generated/ is gitignored.
• Extraction fidelity was machine-verified: prettified files were proven AST-identical to the original runtime strings by byte-comparing esbuild-minified output of both.

Loader

• BuiltinLoader::RunBuiltin is the single compile-and-run path: proper internal/<name>.js script origins (runtime frames are now identifiable in stack traces) and a process-wide bytecode cache — first isolate compiles with kEagerCompile, workers/subsequent isolates consume via kConsumeCodeCache (with rejected-cache fallback), so the ~26KB is parsed once per process instead of once per isolate.
• The placeholder-module proxy is now a static builtin (placeholder-module.js) parameterized with the error message instead of splicing it into source text.

Perf (jitless steady-state)

• Interop::WriteValue no longer recompiles + re-runs an enum's __tsEnum snippet on every FFI marshal — the numeric value is memoized on EnumDataWrapper.
• GlobalPropertyGetter no longer recompiles a JsCode global's snippet on every read — the evaluated result is defined as a real own property on the global (interceptor is kNonMasking, so later reads bypass it entirely). Includes a reentrancy guard: CreateDataProperty re-enters the interceptor before the property exists.

Behavior notes

• The legacy placeholder proxy source never compiled — the spliced error message's quotes made it a guaranteed SyntaxError, so the first require() of a missing optional module always threw via the leaked pending exception (which the shared require tests rely on). CreatePlaceholderModule now keeps that contract explicitly with a proper Error: Cannot find module '<name>'; repeat requires still get the cached placeholder, whose exports now actually throw on access as originally designed.
• Stack traces from runtime internals now show internal/<name>.js origins instead of anonymous <eval> frames.

Testing

• Full TestRunner suite on iOS simulator (rebased on latest feat/error-handling): 903 tests, 0 failures (includes worker tests, which exercise the cross-isolate code cache).
• Incremental build check: codegen phase skipped on no-change rebuild, reruns when a .js input is touched.

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