[EtaCassiopeia]

rift 0.24.0's rift_serve_admin takes scriptsDir. An imposter posted through the embedded admin plane reads lookup CSVs, require() modules and file: scripts only under it, and without it such a lookup is refused (rift#1611, rift#1617). The embedded transport routes replaceAll through that plane, so on the pinned engine an embedded replaceAll carrying a lookup couldn't work at all.

Change

• EmbeddedOptions.scriptsDir(Path) (new), sent as scriptsDir in the serve payload. Like the other serve options, it's refused with EngineUnavailable unless the engine advertises scriptsDir in serveOptions (rift ≥ 0.24.0).
• It does not start the admin plane eagerly. The engine builds each admin request's access from the root when the request arrives, so it doesn't matter when the plane is served. Host-call imposters (create, addStub, applyConfig) stay unconfined (trusted_cwd). The reviewer confirmed both against the v0.24.0 engine source.
• Docs: README (with the spawn twin, SpawnOptions.scriptsDir from chore(deps): bump rift engine to 0.24.0 and write behaviors the way it admits them #346), docs/design/sdk-api.md, Javadoc.

Tests

• ServeOptionsScriptsDirTest (+6): the payload key, absent when unset, refusal naming the setter on a 0.23 build-info, acceptance on 0.24, no eager start, null refused.
• LaunchOptionsIT (+3, real engine), all through replaceAll:
  • a relative CSV path under scriptsDir serves the row;
  • with no scriptsDir the lookup is refused with no --scripts-dir is configured;
  • a CSV outside the root is refused with outside --scripts-dir.

Verification (local, JDK 21, rift 0.24.0)

Full RIFT_IT=1 verify across the reactor (spawn lane, rift-java-embedded-jdk21 with the 0.24.0 dylib, conformance with the 0.24.0 corpus): all modules green.

Downstream: rift-scala's facade needs FacadeCoverage entries for EmbeddedOptions.scriptsDir and SpawnOptions.scriptsDir at its next rift-java bump.

Closes #343

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