feat!: let plugins expose subpath entry points via plugin:<id>/<subpath>

[oglassdev]

A plugin could expose only one module to pack sources, its config, so plugins with a larger authoring API couldn't split it into entry points. Window wants its low-level API at plugin:window/raw beside plugin:window. The # specifiers also read like package-internal imports when they're really provided by the host.

Plugins can now declare extra modules in rpp.json:

{ "config": "src/config.ts", "exports": { "./raw": "src/raw.ts" } }

Pack sources and rpp.config.ts import them as plugin:<name>/<subpath>, and plugin:<name> still means the config module. Every import uses a scheme now:

Before	After
#rpp	rpp
#rpp/config	rpp:config
#rpp/jsx	rpp:jsx
#plugins/<name>	plugin:<name>

This is a breaking change with no alias: #rpp and #plugins/ no longer resolve. Bundle errors for them name the new specifiers. Published plugin archives import #rpp, so they have to be republished (Window included).

Closes #112.

Details

• exports keys must be ./<relative path>. Undeclared subpaths fail in both the bundler and tsc, since codegen writes one exact paths entry per module. Entries that share a package directory bundle shared modules once.
• rpp plugin pack builds each export to dist/exports/<subpath>.js, adds a .d.ts stub and isolated declarations for it, aliases plugin:<self>/<subpath>, and rewrites exports in the packed manifest. Facade chunks (pure re-exports) no longer fail packing for lacking a source map.
• Internal virtual modules moved to rpp:internal/* so they don't sit next to user-facing rpp: modules.
• I tested scheme specifiers through tsconfig paths under moduleResolution: "Bundler" on TypeScript 5 and 7, including rpp:jsx/jsx-runtime.

Validation

• Tests pass for rpp-js, rpp --features js and rpp-cli. New tests cover subpath module sharing, packing exports, importing a subpath from a discovered file, the rename hint, codegen paths and the packed manifest.
• just lint, just fmt-check and just check-features pass. I didn't run verify-jvm locally; the JVM example's import change is covered in CI.

AGENTS.md still says the SDK is imported as #rpp. I left it alone because it shouldn't change without being asked.

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