Engine: bring-your-own-API-key tier (PROPOSAL §4 tier 3) — backlog, unscheduled

[realproject7]

Goal

PROPOSAL §4 defines a third engine tier — bring-your-own Anthropic API key for explicit pay-per-use — deferred at MVP (packages/engine/src/types.ts:3: "BYO-API post-MVP"). This ticket tracks it so the roadmap stays complete. Backlog / unscheduled — do not start without operator prioritization.

Context (verified on main @ fa26d60)

• packages/engine/src/types.ts:85 — TranslationEngine interface both existing tiers implement
• packages/engine/src/env.ts — currently STRIPS ANTHROPIC_API_KEY/ANTHROPIC_AUTH_TOKEN from the CLI child env (to force subscription auth); a BYO tier is a separate direct-API engine, NOT a relaxation of this strip
• Engine selector UI: src/settings-sheet.ts:63-66 (two segments today); pref type src/app-settings.ts:4
• Credit metering: packages/engine/src/credit-ledger.ts (BYO turns carry real per-token cost — the gauge becomes literal)

Sketch (to be re-scoped when scheduled)

Direct Anthropic API engine (streaming, same TranslationEngine contract), key stored in macOS Keychain (never settings JSON/plaintext), third segment in Settings + onboarding, real cost surfaced via the existing gauge.

Security invariants (binding whenever this is built)

Key in Keychain only; never logged, never in argv (see #26 history), never in any file the archive/retention sweep touches.

Routing

Unscheduled backlog. Likely split quadwork (engine) + claude (Keychain/UI) when activated.

Part of #107 (backlog row).