envelope: single MIME implementation — export detection/parse helpers, delete per-app parsers

[rubenhensen]

Today the envelope exists in several hands: pg-js buildMime/extract, Outlook's hand-rolled multipart MIME parser (src/lib/mime.ts), per-app detection logic, and three inconsistent meanings of the x-postguard header (cryptify stamps a pg-core version, addons stamp a hardcoded "0.1.0" marker, decrypt flows stamp "decrypted"). The audit traces multiple cross-client breakages to this duplication (tier-3 detection, data.bin zip asymmetry).

After the app imports land: export detection/parse helpers from packages/pg-js, delete the per-app parsers, and document the header semantics once. Depends on the four import issues.

Child of the monorepo umbrella; part of encryption4all/postguard#247 (workstream B).

[rubenhensen]

Reopened — this was closed by accident, not completed.

The tb-addon import (#137) brought 435 commits in via a subtree merge, and three of them carry closing keywords for #127/#128/#129 — referring to postguard-tb-addon issues. GitHub resolved those against this repo's numbering, so merging the import silently closed B4, B5 and B6.

Guard for the remaining imports, in #139.