fix(benchmarks): split BENCHMARK_HUB_DATA_DIR on the platform path delimiter

[Tyagiquamar]

Problem

BENCHMARK_HUB_DATA_DIR is documented as a multi-root list, but every consumer parsed it by splitting on ":" — the POSIX path-list separator:

• src/benchmark-hub-core.ts (slugRoots)
• src/benchmarks-mcp.ts (configureBenchmarksMcpRoots, hubPrimaryRoot)

On Windows there is no way to spell a colon-separated list of absolute directories: an absolute path like C:\Users\me\.understudy\benchmarks splits into "C" and "\Users\me\.understudy\benchmarks". The first fragment becomes the data root (resolving to a nonexistent <cwd>\C directory) and the real directory lands under the data2 prefix. Every slug lookup then fails with unknown benchmark slug: data--<name> (use list_benchmarks) — for the MCP server, the CLI, and the hub loaders alike.

I found this by running the test suite on Windows: ~31 tests across tests/benchmarks-mcp.test.mjs, tests/experiment-lineage.test.mjs, and tests/benchmark-extension.test.mjs failed with exactly this signature; the fixture sets BENCHMARK_HUB_DATA_DIR to a single absolute temp dir, which is enough to trigger the mis-split.

Fix

Parse and join the env var with path.delimiter (: on POSIX, ; on Windows). Behavior on POSIX is byte-for-byte unchanged; on Windows, single absolute paths now parse correctly and the documented multi-root list works with the platform separator. Also updated the contract comment/README wording and one test assertion that hardcoded the : join.

Testing

On Windows 11, Node 24 (before → after):

node --test tests/benchmarks-mcp.test.mjs tests/experiment-lineage.test.mjs tests/benchmark-extension.test.mjs

before: exit 1 — 31 failures ("unknown benchmark slug: data--promoted", "fixture benchmark discovered through the shared loader", ...)
after:  exit 0 — tests 58, pass 58, fail 0, skipped 0

npm run typecheck
exit 0

[Tyagiquamar]

Hi @lluisinthedesert, gentle review ping when convenient. #471, #472, and #473 are three small platform-compatibility fixes that are ready for review. Happy to make any changes you'd like.