feat(cognition,#1375): check_redundancy PR-1 — pure types + prompt + parser

[joelteply]

Summary

Oxidizer for AIDecisionService.checkRedundancy (TS, see src/system/ai/server/AIDecisionService.ts:165-308). Mirrors the should_respond.rs gating arm + rate_proposals PR-1 shape (#1290).

What this ships (PR-1 scope — pure, atomic)

• RedundancyCheckRequest (ts-rs) — { context: AIDecisionContext, draftText, model? }
• RedundancyDecision (ts-rs) — { isRedundant, reason, model, timestamp }
• ParsedRedundancyResponse — internal parser output (caller in PR-2 stamps model + timestamp)
• RedundancyParseError — typed: NoJsonObject, NotAnObject, MissingIsRedundant
• build_redundancy_prompt(&AIDecisionContext, draft_text) -> String — pure. Embeds last N=10 conversation messages in [HH:MM] speaker: content shape, then draft, then JSON schema.
• parse_redundancy_response(&str) -> Result<...> — pure. Extracts first balanced JSON object, decodes, validates isRedundant boolean.

NOT in this PR

• PR-2: cognition/check-redundancy IPC handler — composes prompt → AI provider call (existing Groq router) → parse → RedundancyDecision with model + timestamp stamped.
• PR-3: TS AIDecisionService.checkRedundancy shim — replaces inline prompt + AIProviderDaemon.generateText with the IPC call.
• PR-4: Delete dead TS code (the inline prompt template + JSON parsing) — same pattern as rate_proposals PR-3 (refactor(cognition,#1289): rate_proposals PR-3 — delete dead TS adapter (no production callers) #1293).

Discipline

• No silent default-on-error. Parser returns typed Result, never panics.
• Caller decides fail-open vs fail-closed — module never invents a default.
• Pure prompt builder uses UTC (removes hidden TZ dependency the TS version's local-time prefix had).
• Snippet bounding on error variants caps upstream garbage in error messages.
• Conversation types reused from gating stack (no new shapes invented for shared concepts).

Tests (18, all passing)

Prompt (6 tests): draft + conversation embed, role fallback, time-prefix omission, last-N window + chronological order, empty conversation, unescaped JSON schema example.

Parser (9 tests): bare JSON, JSON-in-prose, default reason, all error variants, nested object extraction.

Bounds (1 test): snippet bounding (200-byte ASCII prefix + 3-byte UTF-8 ellipsis).

ts-rs (2 tests): export bindings.

Full cognition regression: 292/292 pass.

Refs

• oxidizer: migrate AIDecisionService.checkRedundancy to Rust cognition/check-redundancy #1375 — sub-card just filed (concrete spin from umbrella)
• TS-side AI logic that violates 'TS is thin glue' directive — umbrella triage #1248 — umbrella: TS-side AI logic violates 'TS is thin glue' directive
• Pattern: rate_proposals stack refactor(cognition,#1289): rate_proposals PR-1 — Rust types+prompt+parser slice #1290/refactor(cognition,#1289): rate_proposals PR-2 — IPC handler + orchestrator #1291/refactor(cognition,#1289): rate_proposals PR-3 — delete dead TS adapter (no production callers) #1293 (MERGED)
• Sibling: should_respond.rs (already oxidized — evaluateGating arm)

Test plan

• cargo test --package continuum-core --lib --features metal,accelerate cognition::check_redundancy:: — 18/18 pass
• cargo test --package continuum-core --lib --features metal,accelerate cognition:: — 292/292 pass (no regressions)
• Pre-push TS clean, ESLint baseline held, Rust compile clean
• CI green
• PR-2 IPC handler stack on top (separate PR)

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