h3: accept client trust anchors as PEM text, not only as a path

[MDA2AV]

Mutual TLS over HTTP/3 could only be configured with a client CA file. The TCP side has always taken either form:

public string? ClientCaPath { get; init; }   // a bundle on disk
public string? ClientCaPem  { get; init; }   // the same bundle as text

QUIC took only the path. iq_engine_new_mtls handed it straight to X509_STORE_load_locations, which cannot read memory, so there was nowhere for the text form to go.

The consequence was worse than an inconvenience. A caller holding only the PEM string - a host that keeps its bundle in a secrets store rather than on disk - got mutual TLS on HTTP/1.1 and HTTP/2 and none at all on HTTP/3. Nothing was logged, because from the shim's point of view no client CA had been configured. On an endpoint serving Protocols.All that is the same origin enforcing client certificates on two transports and waving everyone through on the third.

What changed

iq_engine_new_mtls takes client_ca_pem alongside client_ca_pem_path, and builds the same X509_STORE from a BIO when given text, adding each block with X509_STORE_add_cert. A block already present counts rather than failing - a bundle naming one anchor twice is not an error. With both sources null the whole block is skipped exactly as before, so a server that never asked for mutual TLS gets the handshake it had, byte for byte.

QuicEngine refuses both sources at once instead of quietly preferring one, matching what TlsService already does for TCP.

Testing

Three tests added to the h3 mutual-TLS suite, anchors loaded from text in each:

• a trusted client connects and the handler is told who it is
• a certificate from another CA is refused - the one that matters, since a store that verified nothing would pass the first test just as happily
• naming both a file and text is refused

Full suites, all green: E2E 54, Tls 23, Chaos 47, Unit 31, Http 28 - 183 passed, 0 failed.

Also exercised through Playground/Http3/MutualTls, which now takes PLAYGROUND_CLIENT_CA_PEM:

run	result
PEM text, certificate optional	h3x connects anonymously, 5/5
PEM text, certificate required	h3x offers none, 0/5 - refused
malformed PEM text	refused at startup: the client CA PEM text held no usable certificate

Performance

The native library was rebuilt (ngtcp2 + picotls + shim), so the benchable h3 samples were re-measured against their recorded baselines at the same reactor count:

sample	baseline 96137b9	this branch
Http3/Nghttp3Buffered	516,707 req/s	523,782 req/s	+1.4%
Http3/Buffered	665,936 req/s	657,288 req/s	-1.3%

Noise in both directions. The added code runs once, at engine construction, and not on any data path.

No bench result is committed here: the run was made on a dirty tree, and recording that as latest.json is precisely what 9b4579e set out to stop.