[EPIC] Performance engineering + benchmarks

[kirich1409]

Context

Validates performance invariants P1-P10. Uses XCTest performance measurements or swift-benchmark (choice via spike T-18-1).

Goals

• Benchmark suite runnable via swift test --filter Perf*.
• Benchmarks for FastWalker (P1), stream batching (P2), RemovalLogger fsync count (P3), archive streaming hash (P4).
• Results captured (CSV or swift-benchmark snapshots).

Child tasks

Will be added as sub-issues after task batch.

Exit criteria

• AC-30, AC-31, AC-32 pass.
• CI job runs benchmarks on pull-request (advisory, not blocking 0.1).

Related documents

n/a