feat: add Terraform/OpenTofu registry remote type (#45)

## Summary

- New `terraform` package type implementing the [Terraform Registry Protocol](https://developer.hashicorp.com/terraform/internals/provider-registry-protocol)
- `construct_url` prepends `/v1/providers/` so paths like `hashicorp/vault/versions` map to `registry.terraform.io/v1/providers/hashicorp/vault/versions`
- `resolve_content` rewrites `download_url`, `shasums_url`, and `shasums_signature_url` in per-version download info JSON to route through a companion `releases_remote` (generic remote proxying `releases.hashicorp.com`)
- Built-in mutable pattern for `{namespace}/{type}/versions` — version lists expire and are re-fetched; per-version download info is immutable
- Client configuration via `.terraformrc` / `.tofurc` host block — no changes to `.tf` provider source addresses needed

## Test plan

- [x] 8 unit tests covering mutable detection, URL rewriting, binary pass-through, `construct_url` correctness, and cache miss behaviour
- [x] End-to-end: OpenTofu 1.10.3 pulling `hashicorp/vault v4.5.0` through docker-compose stack — `tofu init` succeeded, provider installed and signed
- [x] Verified `download_url` / `shasums_url` rewritten to `hashicorp-releases` proxy in cached response
- [x] All 339 tests pass

Reviewed-on: #45
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

tests/conftest.py

@@ -118,6 +118,21 @@ TEST_REMOTES = {
             "immutable_patterns": [r"^v3/files/.*\.tar\.gz$"],
             "cache": {"immutable_ttl": 0, "mutable_ttl": 600},
         },
+        "terraform-registry-test": {
+            "base_url": "https://registry.terraform.io",
+            "package": "terraform",
+            "immutable_patterns": [
+                r"[^/]+/[^/]+/[^/]+/download/[^/]+/[^/]+$",
+            ],
+            "releases_remote": "hashicorp-releases-test",
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 300},
+        },
+        "hashicorp-releases-test": {
+            "base_url": "https://releases.hashicorp.com",
+            "package": "generic",
+            "immutable_patterns": [r".*\.zip$", r".*SHA256SUMS(\.sig)?$"],
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 0},
+        },
     },
     "locals": {
         "local-test": {