refactor: remove strict segment migration machinery

[duck8823]

Summary

• remove the strict resumable segment-migration protocol and the earlier ledger, sequence inventory, durable planner, reconciliation, and probabilistic summary machinery
• retain and simplify the immutable compressed segment format
• replace migrations 44-48 with one unreleased migration 44 containing a single lightweight archive_segments registry
• verify and fsync an installed sealed segment before idempotent Catalog registration
• reject abandoned development migration-44 identity during ordinary initialization
• document the smaller at-least-once v0.34.0 design

Size reduction

• 12,557 lines removed; 704 lines added
• 75 changed files
• no offline Hot candidate, database-wide atomic exchange, durable migration-run journal, placement ledger/epochs, exact crash resume, post-eviction rollback, archive-sequence inventory, durable target plan, Bloom summary, or reconciliation generation

Retained boundary

• immutable compressed SQLite segment build/read/verify
• small manifest with range, counts, byte/time bounds, and digests
• one lightweight Catalog table for verified installed segments
• active/latest/handoff remain Hot-only
• search routing and eviction remain explicitly outside this cleanup (v0.34.0-27: search Hot and immutable segments #1652/v0.34.0-28: archive and evict one bounded Hot range #1653)

Validation

• go build ./...
• GOCACHE=/private/tmp/traceary-go-cache-1669-final2 go test ./...
• GOCACHE=/private/tmp/traceary-go-cache-1669-final2 GOLANGCI_LINT_CACHE=/private/tmp/traceary-lint-cache-1669-final2 go tool golangci-lint run (0 issues)
• git diff --check

Review evidence

• Design and implementation: Claude Fable 5 (claude-fable-5, effort max)
• Independent Structure-Behavior review: APPROVE
• Initial MUST findings fixed: migration identity drift and unverified raw Catalog registration

Compatibility

• v0.34.0 has not been released; development databases that applied the abandoned migrations 44-48 must be recreated
• existing immutable segment and unrelated Hot behavior remain covered by the full repository suite

Closes #1669