Add pack-index object existence checker strategy for prefetch

[tyrielv]

Summary

Introduces a strategy pattern (IObjectExistenceChecker) to decouple blob prefetch from libgit2's git_revparse_single, which is extremely slow for missing objects (~2.8ms/op with 14 packs in a large GVFS cache).

New PackIndexObjectExistenceChecker reads MIDX and supplemental .idx files directly in managed code via memory-mapped IO (~5μs/op), with loose-object File.Exists fallback. 460x faster for cache-miss lookups.

Gated on config

git config gvfs.prefetch-use-idx true

Default: false (existing revparse behavior unchanged).

Components

File	Purpose
IObjectExistenceChecker	Strategy interface
LibGit2ObjectExistenceChecker	Wraps existing LibGit2Repo.ObjectExists
PackIndexObjectExistenceChecker	MIDX + pack idx + loose fallback
MidxReader	Memory-mapped MIDX v1 parser with binary search
PackIndexReader	Memory-mapped pack index v2 parser with binary search

Design decisions

• Searches both LocalObjectsRoot and GitObjectsRoot (shared cache), de-duplicated
• Detects supplemental packs not yet in MIDX via PNAM chunk diffing
• Thread-safe: shared mmap instance across all FindBlobsStage workers, wrapped in NonDisposingCheckerWrapper
• Falls back to revparse on any initialization error
• 19 new unit tests covering hit/miss, fanout buckets, supplemental packs, loose objects, empty dirs, corrupt files

Benchmark (59.7M objects, 14 packs)

Method	Exists (ns/op)	Missing (ns/op)	Batch 1000 mixed (ms)
git_revparse_single	762	2,801,984	1,796
MIDX binary search	5,371	6,386	2