fix(call-graph): external identity canonicalization edge cases

[rahlk]

Consolidate several external-identity canonicalization gaps found while reviewing the node:<mod> work:

• typesVersions redirect dirs are not normalized: @types/node/ts4.8/fs.d.ts canonicalizes to node:ts4.8/fs instead of node:fs.
• @types/node/globals.d.ts yields a non-specifier node:globals rather than a real module specifier.
• an unprefixed "fs" fallback and the checker's node:fs produce split identities for the same module.
• no test covers the checker-path bare default-import call shape.

Found in the 1.0.0 final review.