Resolve filesystem denials with explicit path context

[copyberry]

Resolve filesystem denials with explicit path context

Why

Filesystem denial paths need to use the owning environment's path syntax, base directory, and home directory. Host-native resolution cannot supply those facts for another platform, and invalid denials must not be silently skipped when building the sandbox policy.

What changed

• Add ConfigPathContext to requirements layers so permissions.filesystem.deny_read can resolve using explicit POSIX or Windows path facts, with native defaults when no context is supplied.
• Share URI-based resolution and validation for literal paths and glob prefixes. Reject ambiguous or lossy paths, including NUL bytes, Windows stream syntax, and unsupported UNC spellings.
• Move denial conversion into FilesystemConstraints::apply_to_policy, preserving glob patterns and deduplicating entries. Validate all denials before modifying the policy and propagate failures through configuration loading.

Testing

Add tests for per-layer base and home resolution, Windows drives and globs, nested context restoration, missing home directories, and policy conversion without partial mutation. Add a configuration-loading regression test for a required denial glob containing a NUL byte.

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