format-stress-test-disagg-switch-tsan failed on ! Amazon2023 ARM64 TSAN

Host: N/A
Project: WiredTiger (Fast Truncate)
Commit: diff: WT-17493 Extract tiered storage thread fields into WT_CONN_TIERED (#13776)

1. 1. Summary

• Introduces `WT_CONN_TIERED` (`struct __wt_conn_tiered`), a new
  sub-struct in `__wt_connection_impl` that owns all tiered-storage thread
  management state
• Moves 13 fields out of the flat connection struct: the tiered work
  queue (`tieredqh`), thread handle (`session`, `tid`, `tid_set`),
  condvars (`cond`, `flush_cond`), timing (`interval`), liveness flag
  (`server_running`), and flush state (`flush_ckpt_complete`,
  `flush_most_recent`, `flush_state`, `flush_ts`)
• Both spinlocks that guard tiered operations (`tiered_lock`,
  `flush_tier_lock`) travel into the sub-struct, following the
  purpose-based grouping convention established in SPM-3483
• Access paths updated from `conn->tiered_` / `conn->flush_` to
  `conn->tiered.<field>` across 8 files; grep confirms no stale references
  remain
• Follows the same struct placement and naming conventions as WT-17491
  (WT_CONN_EXTENSIONS) and WT-17492 (WT_CONN_STAT_LOG)

1. 1. Testing
      Pure refactor — no behaviour change. Verified by:

• Clean `ninja wt` build with no warnings
• Grep across `src/` and `test/` confirms no stale references to old
  field paths

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