add async hydration performance fixes

[elithrar]

• Add async repo prepare/mount flow with gated FUSE readiness and prepared-gitdir safeguards.
• Improve async hydration duplicate-read path: concurrent requests for the same cold blob now join the active fetch instead of refetching.
• Benchmark, faster path: duplicate-read async hydration median improved from 52.8 ms/op to 34.3 ms/op, about 35% faster.
• Benchmark, unchanged path: unique-object real gitstore hydration stayed effectively flat, 359.2 ms/op to 362.8 ms/op median.
• Not a clone/fetch speedup: clone, fetch, snapshot publishing, and unique-object cat-file hydration are not meaningfully faster.
• Harden credential redaction/rejection so inline credentials do not reach git argv; make BlobToCache cancellation kill batch process groups.
• Validation: go build ./cmd/artifact-fs; go vet ./...; go test ./...