fix: resolve tsconfig per file & map package imports from outDir to rootDir

[sahiljadhav7]

Fixes #696, fixes #714, fixes #482

What was wrong

Monorepos (#696, #482). tsx loaded a single tsconfig, found by walking up from the current directory. Files outside that tsconfig got no tsconfig at all, so a workspace package lost its own settings (e.g. jsx: react-jsx → "React is not defined"). Running from a directory without a tsconfig also meant the entry file's own tsconfig was ignored. Worse, the app's paths were applied to other packages' imports. A dependency's ~/lib/label could quietly load the app's ~/lib/label file with no error.

Package imports (#714). With "imports": { "#~/*": "./dist/*" }, tsx looked in dist/ while tsc maps back to src/. With no build you got ERR_MODULE_NOT_FOUND. With an old build lying around, tsx silently ran the stale output.

What changed

• Each file now uses the tsconfig that owns it, the same way the TypeScript language server picks one (getTsconfig(file, { includes: true }), already in get-tsconfig v5).
• Path aliases and allowJs come from the tsconfig nearest the importing file. Scripts outside include, --eval, stdin and the REPL keep resolving aliases just as before.
• --tsconfig / TSX_TSCONFIG_PATH behave exactly as before: one config for the whole run.
• # imports that land in outDir/declarationDir resolve to the rootDir source first, so stale builds are never picked up. Like TypeScript, this only happens when the tsconfig belongs to the importing package and the target isn't in node_modules. Without rootDir, the tsconfig's directory is the source root.
• Both the ESM and CJS loaders are covered.

Tests

Added tests in tests/specs/tsconfig.ts, run for both ESM and CJS packages. They cover:

• workspace dependencies compiled with their own tsconfig;
• running from a directory with no tsconfig;
• the alias hijack case;
• # imports mapped to source, including the stale-build case;
• the new package-scope and node_modules guards;
• behaviour that has to stay the same: --eval, stdin, scripts outside include, explicit --tsconfig, and no rootDir.

Each test failed before its fix. For the compatibility tests, I reintroduced each regression on purpose and checked that the right tests failed.

Full suite locally on Node 24.2.0: the same 8 tests fail on master and on this branch, so nothing new breaks. The failures seem to come from running 24.2 instead of the pinned 24.15.

Notes for review

• Some files outside a tsconfig's include used to get no tsconfig. They now get the tsconfig that actually owns them. This is the intended fix, but it is a visible behaviour change.
• This goes the other way from fix(esm/hook/resolve): tsconfig when in node_modules #768, which drops the node_modules guard on aliases. Per-file lookup covers that use case without applying the root config to every dependency.