feat: implement STCS compaction

[dkharms]

Description

Adds a STCS (Size-Tiered Compaction Strategy) planner that groups sealed fractions into time-bins and selects compaction candidates, and an executor that merges them into a new sealed fraction via k-way merge.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 38.82784% with 167 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.52%. Comparing base (ec96266) to head (fc90905).

Files with missing lines	Patch %	Lines
compaction/planner.go	12.96%	93 Missing and 1 partial ⚠️
compaction/executor.go	22.22%	27 Missing and 1 partial ⚠️
cmd/seq-db/seq-db.go	0.00%	12 Missing ⚠️
fracmanager/fracmanager.go	0.00%	12 Missing ⚠️
fracmanager/fraction_registry.go	0.00%	11 Missing ⚠️
config/validation.go	64.70%	5 Missing and 1 partial ⚠️
compaction/stcs.go	96.29%	1 Missing and 1 partial ⚠️
indexwriter/index.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #430      +/-   ##
==========================================
+ Coverage   70.29%   70.52%   +0.23%     
==========================================
  Files         229      232       +3     
  Lines       18009    18268     +259     
==========================================
+ Hits        12659    12884     +225     
- Misses       4413     4424      +11     
- Partials      937      960      +23     

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