feat(pageserver): create image layers at L0-L1 boundary

[skyzh]

Problem

Previous attempt #10548 caused some issues in staging and we reverted it. This is a re-attempt to address #11063.

Currently we create image layers at latest record LSN. We would create "future image layers" (i.e., image layers with LSN larger than disk consistent LSN) that need special handling at startup. We also waste a lot of read operations to reconstruct from L0 layers while we could have compacted all of the L0 layers and operate on a flat level of historic layers.

Summary of changes

• Run repartition at L0-L1 boundary.
• Roll out with feature flags.
• Piggyback a change that downgrades "image layer creating below gc_cutoff" to debug level.

[github-actions]

8503 tests run: 7932 passed, 0 failed, 571 skipped (full report)

---

Flaky tests (3)

Postgres 17

• test_emergency_mode: release-arm64-without-lfc
• test_storcon_create_delete_sk_down[DeletionSubject.TIMELINE-RestartStorcon.RESTART]: release-arm64-with-lfc

Postgres 14

• test_sharding_split_failures[failure6]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.3% (9062 of 28021 functions)
• lines: 48.6% (79858 of 164178 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
82669fe at 2025-05-28T07:10:24.480Z :recycle:}