Support case-preserving module paths with case-safe cache keys

[thiremani]

Problem

Pluto currently requires lowercase module paths because module paths are also used directly in filesystem cache paths. The mangled ABI can distinguish case, and common repository/module names may contain uppercase characters, but simply relaxing validation would make cache entries collide on case-insensitive filesystems such as the default macOS and Windows filesystems.

This is separate from PR #78 script-root mangling fix: filesystem-derived script and relative-path names can preserve their existing case, while changing the module-path policy requires a deliberate cache-key design.

Desired properties

• Preserve the module path exact spelling in language identity, diagnostics, and mangled symbols.
• Give differently cased paths distinct, deterministic cache keys on case-insensitive filesystems.
• Keep lowercase cache paths stable where practical.
• Use a reversible escape, for example Go uppercase X to !x, or an equivalently collision-resistant encoding.
• Define how literal escape characters, Unicode, and Unicode normalization are handled.
• Version or migrate existing cache entries if the on-disk scheme changes.

Regression coverage

• Differently cased module paths produce distinct cache directories and ABI symbols.
• Cache encoding round-trips exactly.
• Lowercase paths remain unchanged where intended.
• Invalid or ambiguous escape spellings are rejected.
• Behavior is consistent on case-sensitive and case-insensitive filesystems.