mail: add Thunderbird core adapter and explicit Stalwart connector

[erikleblansch]

Executable source integration

• Pinned Thunderbird 157.0 Tools panel and reusable core adapter for explicitly public cooperative-AI input. No automatic email/attachment harvesting; not confidential private-mail inference or an Assist/ThunderAI replacement.
• Independent Stalwart MailExtension with explicit host grant, transient user/app credentials, user-confirmed account/mailbox selection, selected RFC5322-file upload, Email/import and correlated Email/get metadata readback. No send or automatic retries; uncertain completion remains uncertain.
• Explicit source-only preparation of pinned Stalwart Community v0.16.24, unchanged upstream licenses and full archive/inventory hashes. Enterprise defaults are excluded from the recorded future build recipe.
• Narrow source CI using pinned checkout; no automatic server startup, public SMTP, account provisioning, DNS changes or binary download.

Privacy boundary

Random peers must never receive readable private mail. The connector only addresses an owner-controlled or deliberately trusted server. SMTP reception and its queue can expose ordinary non-E2EE mail to the receiving operator; mailbox-body encryption does not solve that. Ciphertext-only peer custody requires encryption of the complete private object before sharing. No confidential ingress or always-available delivery claim.

Verification

13 focused Python tests and 11 offline Node connector tests passed. The original pinned Stalwart archive was prepared and validated: 1714 exact source files, no server executed. The source overlay for Thunderbird was staged; no complete Thunderbird build/native user journey is established.

Still open

Actual Stalwart Community build and isolated native mailbox proof, service/bootstrap/account lifecycle, existing-account delivery binding, actual Signal Protocol sessions and linked devices, encrypted offline custody and durable import acknowledgements, ordinary Internet ingress, and private mail AI. This draft is an integration baseline, not a finished mail client or self-hosted mail service. Compiler staging is subsequent work and is not included in this commit.