security: harden linker argument derivation against package-metadata injection

[proggeramlug]

#467 was a real instance of package metadata flowing into linker arguments and being interpreted as a directive (@scope/pkg parsed by ld64 as a response-file). Generalize the fix.

Mechanism

Single sanitizer applied at every site where package-name / package-path / package-derived-symbol-name flows into a linker arg. Reject leading @, -, /; reject unescaped spaces; reject shell metacharacters; reject characters not in [A-Za-z0-9_./-] for path-shaped args. Add a fuzz suite over crafted package names so future regressions are caught.

Zero runtime cost (compile-time string sanitization).

Acceptance

• Single sanitizer function applied at all derivation sites
• Fuzz suite over crafted package names (leading @, embedded newlines, response-file directives, ld64-specific edge cases)
• Documented threat model: what we sanitize against
• Followup of perry compile/run: scoped package.json name ("@scope/name") passed unescaped to linker, treated as @response-file #467

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Part of the supply-chain hardening series. Zero runtime cost.