Explicit policy precedence at the run boundary + first-class Action::Suppress

[martsokha]

Summary

Two related changes to how policies are layered and how their actions execute:

1. Explicit precedence at the run boundary. Replace the implicit "Vec position = precedence" rule with an explicit, typed precedence on each ref. Callers now declare layering instead of relying on insertion order they could accidentally sort.
2. First-class `Action::Suppress`. Suppress now records suppressed entities in the audit trail and wins over equal-or-lower-priority Redact rules. Previously all non-Redact actions silently dropped the entity with a debug log.

Also folds in cleanup that came out of the design review: `RedactionMapping` slimmed to its useful core, `#[non_exhaustive]` dropped from the strategy/action enums.

Changes

Policy precedence (`72ec208`)

• New `nvisy_ontology::policy::PolicyRef { id: Uuid, precedence: u32 }`. Lower wins (0 = highest precedence; higher numbers layer underneath as defaults). Required field — explicit declaration is the point.
• `EngineInput.policy_ids: Vec` → `EngineInput.policies: Vec`. `NewRun` (`POST /runs`) request body matches.
• `Pipeline.execute` sorts refs by precedence (stable; ties preserve input order) before resolving from cache. `Policies.policies` ends up in true precedence order — index 0 is highest precedence.
• `Policies::all_strategies` doc spells out the tiebreaker rule explicitly: strategy `priority` first, then policy precedence (via Vec position), then within-policy insertion order.

Action::Suppress implementation (`67bfd0f`)

• New `AuditEntryStatus::Suppressed` marks entries for entities matched by an `Action::Suppress` rule. The entry carries the suppressing `policy_id` and the original value but no replacement.
• Evaluator rewrite: collects all matching strategies per entity, then Suppress wins iff `suppress.priority() <= best matching Redact's priority` (or no Redact matches). Compares the priority field directly rather than slice index, so ties go to Suppress regardless of insertion order.
• Applicator skips Suppressed entries at the top of each `build_*_redactions` loop — they never reach the codec.
• Four unit tests cover the precedence rules (suppress-only, suppress-at-higher, suppress-at-equal-tiebreak, redact-at-higher).
• `Action::Review` / `Alert` / `Block` remain stubs; tracking separately.

Slimmed `RedactionMapping` (`67bfd0f`)

• Dropped `original: String` / `replacement: Option` — they were structurally incapable of holding image/audio payloads and held degenerate placeholder strings for non-text modalities.
• Audit values stay on `AuditEntry.value` as before; `RedactionMap` is now a thin entity-to-location index.
• Future blob-backed reversibility extension tracked in Support reversibility for image/audio redactions in the redaction map #151.

Removed `#[non_exhaustive]` (`67bfd0f`)

• Dropped from the four `TextStrategy` / `ImageStrategy` / `AudioStrategy` / `Action` enums. The codec matches them exhaustively; `non_exhaustive` forced silent catch-all arms that hid new variants from compile-time checks.
• Cleared three dead `_ =>` arms in `apply.rs` after the change.

Test plan

• `cargo check --workspace --all-features` — clean
• `cargo test --workspace --all-features` — all green (380+ tests; nvisy-engine grew from 72 to 76 with the new Suppress tests)
• `cargo clippy --workspace --all-features --no-deps` — clean
• `cargo +nightly fmt --all` — clean
• Manual smoke through a multi-policy run once an integration env is available

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