refactor(engine): inline policies on DetectionInput; drop /policies resource

[martsokha]

Summary

• Policies stop being a persisted resource. The caller submits the full inline policy bodies on every `POST /api/v1/detections`; the engine snapshots digests onto the persisted detection record and the audit references rules by `(policy_name, rule_name)` instead of `(policy_id, rank)`.
• `/api/v1/policies[/{id}]` removed (4 endpoints, request/response types, path helper).
• `Policy::id: Uuid` replaced with `Policy::name: HipStr<'static>`; `PolicyRule` gains a required `name: HipStr<'static>`. Audit-heavy passes share refcounts rather than allocating per-entity.
• `RuleRank` deleted entirely — no production code ever compared ranks; positions in the chain are now expressed by name.

Approach

Two commits, no functional drift between them.

1. Inline policies + audit-by-name (commit `e8cd7d28`). New types in `nvisy_engine::policy`:

• `AnyPolicy` — modality-erased sum (Text/Tabular/Image/Audio) mirroring `AnyAudit`. Submitted inline on every detect call.
• `PolicyDigest { name, version }` — header card stored on the persisted detection record so a SOC-2 auditor can render `" v"` without re-uploading the policy bytes.
• `PolicyDecisionRef { policy_name, rule_name: Option<...> }` — stamped on every policy-driven `AuditEntry`. `rule_name` is `None` when the policy's `default_action` fallback fired.
• `validate_policy_namespace` — runs at the top of `Engine::detect` and rejects duplicate policy names within a submission or duplicate rule names within a policy. The audit's identity-by-name guarantee depends on these invariants.

Server-side: `/api/v1/policies[/{id}]` removed; `POST /api/v1/detections` body's `policies: Vec` becomes `Vec`. Registry-side: `register_policy` / `read_policy` / `unregister_policy` / `unregister_all_policies` / `list_policies` / `list_policies_with_summary` / `policy_cache` / `policies_ks` / `ResourceCache` / `ResourceGuard` deleted.

The detect→redact handoff carries the prepared store through an internal `DetectionHandoff` so the redact pipeline can re-evaluate operator specs without the caller re-submitting policies. After a process restart the in-memory record is gone and the redaction can't proceed — that was already the existing semantic; this PR just makes it explicit.

2. Cut clones; tighten the store surface (commit `85c6eaa7`). Follow-up sweep that addresses an audit pass on the first commit:

• `PolicyStore::from_any_policies(Vec)` consumes by value and moves each `Policy` straight into `Arc::new(p)` — no deep policy clones.
• `DetectionRecord` holds `Arc` + `Vec` built once at `register_pending` time. Redact handoff is an `Arc::clone` of the store.
• `DetectionPipeline::register_pending` now consumes `DetectionInput` and returns a `PreparedDetection` that `execute` consumes — the `AnyPolicy` bodies are touched exactly once.
• `SharedData::policies` typed as `Arc`; dead `with_policy` / `with_policies` builders dropped.
• `PolicyStore` public surface reduced to construction + `resolve`; `insert` / `set` / `get` / `len` / `is_empty` / `new` dropped. `TypeMap`-backed dispatch retained so adding a new modality is purely a new `AnyPolicy` arm — no hardcoded fields or per-modality methods.
• `validate_policy_namespace` stops allocating an intermediate `Vec<&str>` per policy.
• `AnyPolicy::name` returns `&str`. `PolicyDigest` / `PolicyDecisionRef` drop unused `Eq` / `PartialEq` derives.
• `nvisy-server` README rewritten to match `nvisy-engine` / `nvisy-codec` style (tagline + Overview prose; no endpoint or feature tables). OpenAPI `policies` tag and the policies mention in `service/mod.rs` docs cleaned up.

Net diff

`-1110 / +519` across 31 files. Six files deleted entirely: `registry/resource_cache.rs`, `handler/policies.rs`, `handler/request/policies.rs`, `handler/response/policies.rs`, `build_policy_store` (was internal). `RuleRank` and the `set` / `insert` / `get` / `len` PolicyStore surface gone.

Test plan

• Four-gate verify clean on `--all-features` (build, clippy `-D warnings`, full test suite ~600 tests, rustdoc `-D warnings`).
• cargo-deny clean.
• No-default-features build still works (`-p nvisy-codec --no-default-features --features txt`).
• One TOML fixture in `crates/nvisy-engine/tests/redaction_policy.rs` updated to add `name` fields on rules — every other test passes unchanged because they never constructed `DetectionInput::policies` directly.
• 4 new unit tests on `validate_policy_namespace` cover empty input, duplicate-policy rejection, duplicate-rule rejection, and the happy path.