zenhttp improvements (robustness / correctness) (#968)

A collection of security, correctness, and robustness fixes in `zenhttp` and `zencore` surfaced by security review. Most items are small, independent commits grouped here because they all tighten trust boundaries or fix UB along the same code paths.

## WebSocket protocol hardening (RFC 6455)

- **Enforce the client-side mask bit**. Server-side frame loops now reject unmasked frames with close code 1002 per §5.1. Prevents HTTP intermediary smuggling.
- **Validate control frames and RSV bits**. Fragmented control frames, oversized (>125 B) control payloads, and any non-zero RSV bit now fail the connection before allocation.
- **Lower per-frame payload cap** from 256 MB → 4 MB. Bounds per-connection accumulator memory.
- **Implement message fragmentation**. Continuation frames are coalesced and delivered as a single message; interleaved non-control frames close with 1002; assembled messages are capped at 4 MB (1009 on overflow). Previously partial fragments were delivered to handlers, bypassing payload validation.
- **Parse the 101 handshake response properly** in `HttpWsClient`. Status-line, `Upgrade`, `Connection`, and `Sec-WebSocket-Accept` are now matched exactly rather than via substring searches against the full body.

## Auth / OIDC hardening

- **Constant-time password compare** in `PasswordSecurity::IsAllowed` (closes a remote length/content timing oracle). Adds a shared `ConstantTimeEquals` helper.
- **Harden Basic-auth header parsing**: trim trailing LWS, reject control bytes and DEL in the credential.
- **OIDC discovery pinning**: require HTTPS (loopback exempt), verify `issuer` matches `BaseUrl`, require `token_endpoint` / `userinfo_endpoint` / `jwks_uri` to share origin with `BaseUrl`, reject empty `token_endpoint`.
- **Restrict `POST /auth/oidc/refreshtoken`** to local-machine requests. Previously unauthenticated in default deployments — remote callers could evict or replace cached tokens.
- **Stop logging OIDC provider response bodies** on refresh failure (IdPs echo `refresh_token` back in error bodies).
- **Drop the unused `IdentityToken` field** from `OidcClient` / `OpenIdToken` so nothing in the tree accidentally trusts an unverified JWT.

## Auth state encryption migration

- Add `AesGcm` AEAD primitive (BCrypt / OpenSSL backends, mbedTLS stubbed) and `CryptoRandom::Fill` CSPRNG helper in `zencore/crypto.h`.
- Migrate authstate file from AES-256-CBC with a fixed IV to AES-GCM with a fresh 12-byte random nonce per write and the 4-byte `ZEN1` magic bound as AAD. Legacy-CBC files are transparently read once and rewritten in the new format.

## Filesystem / IO robustness

- `IoBufferExtendedCore::Materialize` now checks `MAP_FAILED` on POSIX (was comparing to `nullptr`, which let the failure sentinel propagate into later reads and `munmap(MAP_FAILED, ...)`).
- `IoBufferBuilder::MakeFromFile / MakeFromTemporaryFile`: close the FD/HANDLE on exception via a dismissable `ScopeGuard`; actually check the `fstat()` return value (previously used an uninitialized `FileSize`).
- `ReadFromFileMaybe`: loop short reads, retry `EINTR`, chunk Windows `ReadFile` at `0xFFFFFFFF` bytes (fixes silent truncation of multi-GiB reads).
- `WipeDirectory`: compare `FindFirstFileW` handle against `INVALID_HANDLE_VALUE` rather than `nullptr`.
- `RemoveFileNative` (Linux/macOS): report non-`ENOENT` stat failures via the `std::error_code` out-param and stop reading `st_mode` after a failed stat.

## Buffer / compression correctness

- Avoid per-copy `IoBufferCore` heap allocations in `CompositeBuffer::CopyTo / ViewOrCopyRange` iterators; add fast path for `BufferHeader::Read` when the 64-byte header fits in the first plain-memory segment.
- `BufferHeader`: add `IsHeaderValid()` gate covering `BlockSizeExponent` range, `BlockCount * BlockSize` overflow, and `TotalRawSize` bounds before any arithmetic uses them. Defends against attacker-controlled headers that can pass the CRC and trigger OOB writes in `DecompressBlock`.

1 files changed, 25 insertions, 6 deletions

diff --git a/src/zenserver/storage/cache/httpstructuredcache.cpp b/src/zenserver/storage/cache/httpstructuredcache.cpp
index 4d3673e70..7ca496dec 100644
--- a/src/zenserver/storage/cache/httpstructuredcache.cpp
+++ b/src/zenserver/storage/cache/httpstructuredcache.cpp
@@ -70,6 +70,7 @@ namespace {
 	static constinit std::string_view HttpZCacheUtilStopRecording	= "exec$/stop-recording"sv;
 	static constinit std::string_view HttpZCacheUtilReplayRecording = "exec$/replay-recording"sv;
 	static constinit std::string_view HttpZCacheDetailsPrefix		= "details$"sv;
+	static constinit std::string_view HttpZCacheRecordingsDirName	= "recordings"sv;
 }  // namespace
 
 //////////////////////////////////////////////////////////////////////////
@@ -396,17 +397,26 @@ HttpStructuredCacheService::HandleRequest(HttpServerRequest& Request)
 	{
 		HttpServerRequest::QueryParams Params = Request.GetQueryParams();
 
-		std::string RecordPath = UrlDecode(Params.GetValue("path"));
+		const std::string RecordPath = UrlDecode(Params.GetValue("path"));
+
+		const std::filesystem::path			 RecordingsRoot = m_CacheStore.GetBasePath() / HttpZCacheRecordingsDirName;
+		std::optional<std::filesystem::path> ResolvedPath	= ResolveSafeRelativePath(RecordingsRoot, RecordPath);
+		if (!ResolvedPath)
+		{
+			m_CacheStats.BadRequestCount++;
+			ZEN_WARN("cache RPC start-recording rejected unsafe path '{}'", RecordPath);
+			return Request.WriteResponse(HttpResponseCode::BadRequest);
+		}
 
 		{
 			RwLock::ExclusiveLockScope _(m_RequestRecordingLock);
 			m_RequestRecordingEnabled.store(false);
 			m_RequestRecorder.reset();
 
-			m_RequestRecorder = cache::MakeDiskRequestRecorder(RecordPath);
+			m_RequestRecorder = cache::MakeDiskRequestRecorder(*ResolvedPath);
 			m_RequestRecordingEnabled.store(true);
 		}
-		ZEN_INFO("cache RPC recording STARTED -> '{}'", RecordPath);
+		ZEN_INFO("cache RPC recording STARTED -> '{}'", *ResolvedPath);
 		Request.WriteResponse(HttpResponseCode::OK);
 		return;
 	}
@@ -435,7 +445,16 @@ HttpStructuredCacheService::HandleRequest(HttpServerRequest& Request)
 
 		HttpServerRequest::QueryParams Params = Request.GetQueryParams();
 
-		std::string RecordPath = UrlDecode(Params.GetValue("path"));
+		const std::string RecordPath = UrlDecode(Params.GetValue("path"));
+
+		const std::filesystem::path			 RecordingsRoot = m_CacheStore.GetBasePath() / HttpZCacheRecordingsDirName;
+		std::optional<std::filesystem::path> ResolvedPath	= ResolveSafeRelativePath(RecordingsRoot, RecordPath);
+		if (!ResolvedPath)
+		{
+			m_CacheStats.BadRequestCount++;
+			ZEN_WARN("cache RPC replay-recording rejected unsafe path '{}'", RecordPath);
+			return Request.WriteResponse(HttpResponseCode::BadRequest);
+		}
 
 		const uint32_t HardwareConcurrency = GetHardwareConcurrency();
 		const uint32_t MaxThreadCount	   = std::max<uint32_t>(HardwareConcurrency, 16u);
@@ -449,9 +468,9 @@ HttpStructuredCacheService::HandleRequest(HttpServerRequest& Request)
 		}
 		ThreadCount = std::clamp<uint32_t>(ThreadCount, 1u, MaxThreadCount);
 
-		ZEN_INFO("initiating cache RPC replay using {} threads, from '{}'", ThreadCount, RecordPath);
+		ZEN_INFO("initiating cache RPC replay using {} threads, from '{}'", ThreadCount, *ResolvedPath);
 
-		std::unique_ptr<cache::IRpcRequestReplayer> Replayer(cache::MakeDiskRequestReplayer(RecordPath, false));
+		std::unique_ptr<cache::IRpcRequestReplayer> Replayer(cache::MakeDiskRequestReplayer(*ResolvedPath, false));
 		ReplayRequestRecorder(RequestContext, *Replayer, ThreadCount);
 
 		ZEN_INFO("cache RPC replay COMPLETED");