salience: add gRPC client + TLS plumbing for stateful vllm sessions

Adds the client-side of a stateful gRPC protocol against vllm, plus
the TLS trust machinery so we can talk to self-signed vllm servers.

Protocol (proto/salience.proto):
  Bidi-streaming Session RPC carries OpenSession / AppendTokens /
  Generate / Cancel from client and SessionReady / PrefillProgress /
  Token / GenerateDone / Error from server. Separate Fork unary RPC
  for cheap branching (prefix cache shares KV automatically). Plus
  ListSessions, CloseSession, GetReadoutManifest admin RPCs.

  Per-token readouts ship as packed f32 ([n_layers * n_concepts] per
  token, flat). Logprobs use range-selected positions plus a top-k
  parameter — empty ranges means no logprobs, any range means emit
  sampled-token logprob at those positions, top_k > 0 adds
  alternatives.

Client (src/agent/api/salience.rs):
  Tonic-generated types under pb::, a connect() helper, with_auth()
  for bearer metadata, and a Session handle wrapping the bidi stream:
  open() handshakes SessionReady; append() is fire-and-forget;
  generate() returns impl Stream<Item = Event> that drains inbound
  until Done or terminating Error. One generate at a time per session.

Peak picker (src/agent/salience.rs):
  Pure function over ReadoutEntry traces. Per-concept z-score against
  trace global stats; contiguous above-threshold regions emit one
  peak at the local max. Configurable sigma threshold and min-std
  safety floor. Deterministic tie-break on offset then concept name.
  12 unit tests covering empty traces, flat channels, single/multi
  spikes, contiguous humps, multi-concept independence, trailing
  runs, sub-threshold noise, layer-out-of-range, manifest shape
  mismatch, and threshold tunability.

TLS (src/agent/api/http.rs):
  HttpClient::build now also loads every .pem file under
  ~/.consciousness/certs/ into the rustls root store — so dropping
  a <host>.pem in that directory is enough to trust a new self-
  signed server; no code changes per new host. Also installs the
  rustls default crypto provider explicitly via OnceLock: tonic's
  tls features pulled in both ring and aws-lc-rs on the resolver
  path, and rustls 0.23 refuses to auto-pick when either could win.

Build (build.rs, Cargo.toml):
  tonic-build generates Rust types from proto/salience.proto at
  cargo-build time, using a vendored protoc binary
  (protoc-bin-vendored) so no system install is required. New
  runtime deps: tonic, prost, async-stream, tokio-stream,
  rustls-pemfile.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/mod.rs

@@ -756,6 +756,11 @@ fn restore_stderr(original_fd: std::os::fd::RawFd) {
 
 #[tokio::main]
 pub async fn main() {
+    // Install target-routed file logger: `target: "grpc"` records go to
+    // ~/.consciousness/logs/daemon/grpc.log, everything else to debug.log.
+    // Level from RUST_LOG, defaulting to info.
+    let _ = crate::logging::init();
+
     // Reap channel-daemon zombies via a SIGCHLD handler that only touches
     // PIDs listed in channels_dir(). Avoids SIGCHLD=SIG_IGN, which would
     // break tokio::process::Command::wait() (kernel auto-reap → ECHILD).