REL symbolic workspace roots

RBE recognizes two symbolic roots for host-backed workspace capabilities:

• $$/ — frozen ProjectRoot;
• ??/ — execution-scoped temporary Workspace root.

These spellings are part of the REL capability path language. They are not filesystem paths exposed directly to user code. Backend/Container resolves them against trusted root authority established at boot/execution time.

Examples:

script.run("$$/scripts/build.ts");
archive.create("??/dist", "??/release.zip");

Rules:

• no .. traversal;
• no absolute path suffix;
• no NUL;
• no current-directory fallback;
• no raw OS temp-directory fallback;
• workspace cleanup must not affect ProjectRoot;
• path resolution must remain stable across Windows/Unix separators.