Epic: pageserver sharding tech debt

[jcsp]

### Tasks
- [ ] Compaction: figure out if gap detection needs to be made stripe-aware to avoid spuriously splitting layers
- [ ] https://github.com/neondatabase/neon/issues/5953
- [ ] initdb uploads: these are written in a shard-naive path.  Make sure we have a sharded deletion test that checks they're cleaned up.
- [ ] TenantHarness should use a sharding-enabled state by default for unit tests
- [ ] neon_fixtures.py should use explicitly-set shard by default, even for default unsharded scenario.
- [ ] https://github.com/neondatabase/neon/pull/6037
- [ ] https://github.com/neondatabase/neon/issues/6072
- [ ] https://github.com/neondatabase/neon/pull/6280
- [ ] https://github.com/neondatabase/neon/issues/6307