feat(fs): add direct mail identity primitives

[TZZheng]

Summary

• add unread-independent primitives for direct-mail target identity and deterministic address fingerprints
• normalize To endpoints across scalar/list representations with trimming, empty removal, stable ordering, and literal deduplication
• reuse that normalizer in network topology construction and remove the superseded private recipient parser

Why

Issue #645 needs a stable filesystem-level definition of a direct one-to-one mail target before persistence and UI behavior can build on it. This PR isolates that contract so it can be reviewed and merged independently.

Test-first evidence

TestBuildNetworkDeduplicatesLiteralRecipientsPerMessage was added and run against the exact base before production changes. A message with To: ["bob", "bob"] failed because it contributed edge count 2 instead of 1. The implementation then made that focused regression and the new identity-helper tests pass.

Validation

• go test ./internal/fs
• go test .
• exact reviewed HEAD: e6ae4c2f057762a11449be35023ddae6cb9d84e6
• independent exact-head Claude review: PASS
• independent exact-head Codex review: PASS

Scope

This PR intentionally does not add SessionCache persistence, durable unread state, or TUI behavior. Those responsibilities remain separate follow-up changes.

Part of #645.