fix(results): split mix partitions by a committed durations file

[calvin-archastro]

Merge order: independent of #148; merge either first. #148's guide section "Balanced partitions" is updated to describe this file, so merge this before or with #148.

Fixes the open risk in #149 (T24 of test tracking): a shard that could not reach aster-srv fell back to Mix's split while the others balanced, so files ran on two shards or none.

What changes

before:  shard ──GET /durations──▶ aster-srv     (per shard, can fail alone)
after:   scheduled job: aster results durations ──▶ aster-durations.json ──▶ PR
         shard ──reads──▶ aster-durations.json   (same bytes on every shard)

1. Shards read aster-durations.json at the workspace root, keyed by suite directory (services/platform). No network at run time; .aster/durations/ cache and commit-day lookup removed.
2. aster results durations [--url] finds every MIX_TEST_PARTITION=K mix test --partitions N suite, fetches medians from GET /v1/test-results/durations, writes the file sorted and pretty (clean diffs). Any fetch failure leaves the file unchanged and exits 1.
3. Fallbacks are uniform: no file, suite missing, malformed or newer-version file, ASTER_BALANCE_PARTITIONS=off → Mix's own split on every shard.
4. Docs: server README explains the file and gives a weekly refresh workflow; spec T24 updated.

Verification

• tests/test_results.rs::shards_split_by_the_committed_durations_file_without_a_server: real binary, fake mix on PATH and fake server. No file → Mix split; aster results durations writes the file (asserts the request targets); then both shards run disjoint file lists with no server configured.
• Unit tests: file round trip, broken/newer file is an error, missing credentials, suite discovery, every fallback.
• fmt, clippy -D warnings, rustdoc clean; workspace tests pass except dev_services::dynamic_port_bundles…, which fails locally on main too.

Rollout (firstlanding)

1. Release aster, bump ASTER_VERSION.
2. Run aster results durations once and commit aster-durations.json; add the scheduled refresh workflow.
3. Until the file lands, shards keep Mix's split (no regression).

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