vfs: VFS diverges from the installer in ways that break environments

[baszalmstra]

• No shebang rewriting — text_ranged_read does plain byte substitution; the installer rewrites to #!/usr/bin/env … when the shebang exceeds 127 bytes (Linux) / 512 (macOS) or the prefix contains a space. Long or spaced mount paths yield kernel-unusable shebangs. The added shebang_length field that was meant to support this is dead code — only ever set to None, never read.
• No clobbering resolution — duplicate paths across packages become duplicate readdir entries; the winner is nondeterministic because packages are collected via JoinSet::join_next().
• Windows entry points wrong — hardcoded ./bin + for_windows=false, so noarch-python entry points on the flagship ProjFS path land in the wrong dir as non-executable Unix scripts (installer uses Scripts/ + .exe launcher).
• pypi packages silently dropped — build_metadata_tree only handles as_binary_conda(); Pypi lock entries vanish, yet compute_env_hash counts them. Silently incomplete environment.
• Windows binary/backslash divergence — installer skips binary replacement on Windows and converts \→/ in the prefix; the VFS does neither.