Add invalid capsule conformance vectors and tests

15 negative test vectors across 5 error categories:
- missing_field: no id, type, trigger, reasoning, empty object
- wrong_type: string sequence, string confidence, null trigger.type, array trigger
- invalid_value: negative sequence, confidence >1.0, unknown CapsuleType
- chain_violation: genesis with previous_hash, non-genesis without
- integrity_violation: tampered content with original hash

33 Python tests validate all vectors including hash mismatch
detection via compute_hash(). Coverage guard ensures no
fixture goes untested.

Also updates CHANGELOG [Unreleased] and conformance README.

Author: bradleygauthier

CHANGELOG.md

@@ -9,6 +9,13 @@ Versioning follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Added
+
+- **Go verifier library** (`capsule-go`) -- canonical JSON serialization, SHA3-256 hashing, Ed25519 signature verification, and structural/full/signature chain verification in Go. Passes all 16 golden conformance vectors. Uses `crypto/ed25519` (stdlib) and `golang.org/x/crypto/sha3`. Verification-only (no capsule creation).
+- **LiteLLM integration** (`capsule-litellm`) -- `CapsuleLogger` callback that seals every LLM call into a Capsule. Sync and async support. Captures prompt hash (SHA3-256), token metrics, latency, model identity, and error tracking. Install: `pip install capsule-litellm`.
+- **Invalid capsule fixtures** (`conformance/invalid-fixtures.json`) -- 15 negative test vectors across 5 error categories: missing required fields, wrong types, invalid values, chain violations, and content tampering. Verifiers SHOULD reject all of these.
+- **Python tests for invalid fixtures** (`test_invalid_fixtures.py`) -- 27 tests validating the invalid fixture suite: structure, missing fields, wrong types, invalid values, chain violations, integrity violation via `compute_hash()`, and coverage guard.
+
 ---
 
 ## [1.3.0] - 2026-03-09

conformance/README.md

@@ -99,6 +99,38 @@ The URI spec is at [`spec/uri-scheme.md`](../spec/uri-scheme.md).
 
 ---
 
+## Invalid Capsule Fixtures
+
+The `invalid-fixtures.json` file provides 15 test vectors for **malformed or structurally invalid** capsules. A conformant verifier SHOULD reject each of these.
+
+Each entry contains:
+
+| Field | Type | Description |
+|---|---|---|
+| `capsule_dict` | object | A malformed capsule |
+| `expected_error` | string | Error category: `missing_field`, `wrong_type`, `invalid_value`, `chain_violation`, `integrity_violation` |
+| `error_field` | string | Which field caused the error |
+
+### Error categories
+
+| Category | Description | Example |
+|---|---|---|
+| `missing_field` | A required field is absent | No `id`, no `trigger` section, empty object |
+| `wrong_type` | A field has the wrong JSON type | `sequence` is a string, `trigger` is an array |
+| `invalid_value` | A field has an invalid value | Negative sequence, confidence > 1.0, unknown CapsuleType |
+| `chain_violation` | Chain rules (CPS Section 4) are violated | Genesis with previous_hash, non-genesis without it |
+| `integrity_violation` | Stored hash does not match content | Tampered domain with original hash |
+
+### Invalid capsule conformance check
+
+For every fixture:
+
+1. Attempt to validate the capsule
+2. Confirm the validator rejects it with an appropriate error
+3. For `integrity_violation` fixtures, verify that `VerifyHash(capsule_dict, claimed_hash)` returns false
+
+---
+
 ## Adding New Fixtures
 
 New fixtures must be added through the [protocol change proposal](https://github.com/quantumpipes/capsule/issues/new?template=spec-change.md) process. Every new fixture must: