feat(compaction): wire real CompactManager into MergeTreeWriter for key-table compaction

[Copilot]

Prior to this change, MergeTreeWriter held no CompactManager and its Compact()/Sync()/CompactNotCompleted() methods returned NotImplemented. Key-value tables had no compaction at the framework layer.

Core new components

• MergeTreeCompactManager — orchestrates compaction for KV tables: picks units via CompactStrategy, submits MergeTreeCompactTask to executor, applies results back to Levels
• MergeTreeCompactTask — handles the rewrite-vs-upgrade decision per section of sorted runs; uses IntervalPartition to detect overlapping files; supports DeletionFileSupplier for DV-enabled tables
• FileRewriteCompactTask — simpler per-file rewrite task for lookup-mode full compaction
• CancellationController — atomic bool wrapped in a shared object; passed into all rewriters so in-flight compaction can be cancelled on Close()

Interface changes

• CompactManager::AddNewFile() now returns Status (was void); all implementors updated
• MergeTreeWriter ctor gains a CompactManager parameter; Compact/Sync are now real implementations that flush buffered data, trigger compaction, and sync results
• LookupLevels::levels_ promoted from unique_ptr to shared_ptr so it can be shared with MergeTreeCompactManager
• MergeTreeCompactRewriter and subclasses gain a CancellationController parameter
• WriteContext / AbstractFileStoreWrite / KeyValueFileStoreWrite gain IOManager and a separate key_comparator_for_compact (view-free, for min/max key comparison in Levels)

New config options

compaction.max-size-amplification-percent  (default 200)
compaction.size-ratio                      (default 1)
num-sorted-run.compaction-trigger          (default 5)
num-sorted-run.stop-trigger                (default trigger+3)
num-levels                                 (no default)
compaction.force-up-level-0               (default false)
lookup-compact                             (RADICAL | GENTLE, default RADICAL)
lookup-compact.max-interval               (default 10)

Tests

• MergeTreeCompactManagerTest — 13 unit tests covering level routing, full compaction, stop-trigger enforcement, and reentrant-full-compaction rejection
• KeyValueCompactionInteTest — integration tests for IO-exception recovery and stream-write full compaction
• Existing MergeTreeWriterTest / LookupMergeTreeCompactRewriterTest updated to pass NoopCompactManager where the new ctor requires a manager

TODO: lookup-mode compaction framework is not yet wired (CreateLookupRewriter stubs exist but the full path is incomplete).

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