Introduce IObjectExistenceChecker strategy pattern 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 (~5us/op), with
loose-object File.Exists fallback. Gated on gvfs.prefetch-use-idx
git config (default: false).

Components:
- IObjectExistenceChecker: strategy interface
- RevParseObjectExistenceChecker: 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
- FindBlobsStage: accepts optional checker factory (backward compatible)
- BlobPrefetcher: reads config, creates appropriate checker factory

Searches both LocalObjectsRoot and GitObjectsRoot (shared cache),
detects supplemental packs not yet in MIDX via PNAM chunk diffing,
and safely falls back to revparse on initialization errors.

Unit tests cover: MIDX/idx hit and miss, all 256 fanout buckets,
supplemental pack detection, loose objects, empty/missing pack dirs,
multiple object roots, corrupt file handling, and deduplication.

Assisted-by: Claude Opus 4.6
Signed-off-by: Tyrie Vella <tyrielv@gmail.com>