fix: Unit 1 — publishing blocker + convention fixes (R-01, R-02, R-03)

- R-01: remove `readme = "README.md"` from Cargo.toml (no README yet;
  the field will be restored when the README is written as the
  second-to-last step before publishing). `cargo publish --dry-run
  --allow-dirty` now succeeds.
- R-02: replace the three `expect` calls in non-test library code per
  AGENTS.md §2:
  - `channels/operations.rs` `register_openable`: collapse the
    `is_none()` early-return + `expect("checked above")` into a
    single `ok_or_else(...)?` (same error message, no expect).
  - `core/types.rs` `StreamBidiStreamSource::accept_bi` and
    `close`: use `unwrap_or_else(|e| e.into_inner())` for the
    poisoned `std::sync::Mutex` (per the convention, a panic in one
    operation must not cascade).
- R-03: remove the unnecessary `#[allow(dead_code)]` from
  `registry::context::generate_request_id` — it is called from
  `registry::env`, `protocol::connection` (3 sites), and tests.

The review's R-02 acceptance-gate grep is line-level
(`grep -v 'mod tests'` filters by line, not scope), so the ~269
`.expect(` calls inside `#[cfg(test)] mod tests` blocks still match
it. The intent of R-02 — no `expect` in library (non-test) code — is
met: the three non-test sites identified in the review are removed, and
a targeted grep for the removed strings returns no matches.

Verification:
  cargo test                                    → 483 passed, 0 failed
  cargo clippy --all-targets -- -D warnings     → clean
  cargo fmt --check                             → clean
  cargo doc --no-deps                           → clean (0 warnings)
  cargo publish --dry-run --allow-dirty         → succeeds

src/channels/operations.rs

@@ -405,14 +405,17 @@ impl ChannelCore {
     ) -> Result<(), String> {
         let op_type = spec.op_type;
         let channel_open = spec.channel_open.clone();
-        if channel_open.is_none() {
-            return Err(format!(
-                "register_openable: spec `{}` has no channel_open marker — \
-                 use `OperationSpec::with_channel_open` to set it (ADR-047 §2)",
-                spec.name
-            ));
-        }
-        let alpn = channel_open.expect("checked above").alpn.to_string();
+        let alpn = channel_open
+            .as_ref()
+            .ok_or_else(|| {
+                format!(
+                    "register_openable: spec `{}` has no channel_open marker — \
+                     use `OperationSpec::with_channel_open` to set it (ADR-047 §2)",
+                    spec.name
+                )
+            })?
+            .alpn
+            .to_string();
 
         let manager = self.manager.clone();
         let policy = Arc::clone(&self.policy);