Fix unknown-size file attrs

[elithrar]

Fixes #16. Unknown-size blobless files should not be exposed through FUSE as real zero-byte files.

Issue #16 reports that after batch size resolution fails, tracked blobless files can stat as 0 bytes, cat returns no content, and git status marks them modified before prefetch catches up.

• Hydrate unknown-size base file attrs during lookup and cached getattr
• Keep known-size, overlay, directory, and symlink attrs on the cheap metadata path
• Preserve GIT_NO_LAZY_FETCH=1 and retain partial batch size results if cat-file exits after writing stdout
• Return EIO for attr hydration failures instead of making existing files look deleted

User-facing result:

stat packages/wrangler/src/index.ts
cat packages/wrangler/src/index.ts
git status --short --untracked-files=no packages/wrangler/src/index.ts

The target file now reports its hydrated size, reads its Git blob content, and remains clean once hydration succeeds.

Tests passed:

• go build ./cmd/artifact-fs
• go vet ./...
• go test ./...
• go test ./internal/fusefs ./internal/gitstore
• AFS_RUN_BENCH=1 go test -run TestBenchRepos -v
• AFS_RUN_E2E_TESTS=1 go test -run '^TestE2E$' -v .\n- AFS_RUN_E2E_TESTS=1 go test -run TestE2E -v . passed on retry