zenhttp improvements (robustness / correctness) (#968)HEADmain

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`.

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