Hardlink resident layers during detach ancestor

[arpad-m]

After a detach ancestor operation, we don't want to on-demand download layers that are already resident. This has shown to impede performance, sometimes quite a lot (50 seconds: #8828 (comment))

Fixes #8828.

[github-actions]

7425 tests run: 7045 passed, 0 failed, 380 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_pageserver_gc_compaction_idempotent[after_restart]: release-arm64-with-lfc
• test_scrubber_physical_gc_ancestors[None]: release-arm64-without-lfc
• test_scrubber_physical_gc_ancestors[2]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 33.2% (8586 of 25850 functions)
• lines: 49.1% (72308 of 147327 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
18456d3 at 2025-02-11T16:13:04.967Z :recycle:}

[problame]

Preexiting nit: I think adoptee is the wrong term? The adoptee is the detached Timeline, hence it is _adopting the layer, hence adopter would be more appropriate.
Please flag if my understanding is incorrect, but let's leave the naming as is to keep this patch on point.

[arpad-m]

Preexiting nit: I think adoptee is the wrong term? The adoptee is the detached Timeline, hence it is _adopting the layer, hence adopter would be more appropriate.

yeah you are right, this was confusing to me as well.