revert: remove access-frequency ILM tiering (#60)

Reverse the first-parent diff of a3df317ae0,
including the feature branch compatibility and mover follow-up fixes.
Retain the independent multi-pool correctness fixes from #178 and migrate
their shared test fixture away from access-tier code.

Tolerate retired ILM keys and XML, read old v9 statistics while writing v8,
and document migration without moving objects or rewriting their metadata.
Include regression coverage using a historical scanner/writer v9 fixture.

Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/testdata/data-usage-v9/README.md

@@ -0,0 +1,27 @@
+# Data-usage v9 retirement fixture
+
+Generated using unmodified `scanDataFolder` and `dataUsageCache.serializeTo` from
+Server commit `89637554d60c27cfc51d2281d0a4fe15e415f06d` (Go 1.27.1, darwin/arm64).
+The scanner visits three real local files; its size callback supplies synthetic
+version/delete-marker/remote-tier summaries, following `TestDataUsageCacheSerialize`.
+It is a scanner/cache compatibility fixture, not a distributed object-store test.
+
+The old scanner counts 74,962 bytes, 3 objects, 5 versions and 2 delete markers.
+Its nonzero retired `hts` totals 9,426 bytes. Remote tier `COLD` contains 65,536
+bytes, one version and one object. The cache includes nested children, both
+histograms, a fixed timestamp and scanner cycle 42. The JSON is the old scanner's
+complete expected cache (including `HotTierSize`, which the new reader ignores).
+
+Binary SHA-256: `4c9c7e94cea7758fe9b498b19f639188764b2787582783e6cc950f2c44d52a8b`.
+
+To regenerate, create a detached worktree at that exact source commit, copy
+`generate.go.txt` to `cmd/retirement-fixture_test.go`, and run:
+
+```sh
+SILO_RETIRE_FIXTURE_DIR=/absolute/output/directory go test ./cmd -run '^TestGenerateAccessRetirementV9Fixture$' -count=1 -v
+```
+
+The historical production writer adds the version byte, compresses with zstd
+and encodes msgp, including the nonzero `hts` field. Do not regenerate using the
+retired implementation or by changing the header of a v8 payload. Map order may
+change serialized bytes across regeneration; compare the decoded full cache.