fix: address code review #001 findings (M1, M2, L2, L4, L5, N1-N3, N5)

- M1: TtySession now handles the negotiation-rejection error frame.
  from_halves peeks the first response byte (ADR-052 §5 disambiguation)
  and returns NegotiationRejected on a 0x00-prefixed error frame;
  ChunkReader gains peek_stream_type/read_chunk_after_peek.
- M2: wait() now surfaces MalformedExitChunk instead of collapsing it
  to NoExitChunk. The exit watch channel carries a cloneable
  ExitOutcome enum; MalformedExitChunk carries a String.
- L2: add EmittingBackend + recv_stdout_and_stderr_route_backend_data
  test covering the consumer read-pump stdout/stderr routing.
- L4: MockBackend/MockControl/MockStdinSink are now #[cfg(test)]
  pub(crate), removing them from the public API.
- L5: cargo fmt (the BAST drift test was unformatted).
- N1: fix all 9 rustdoc intra-doc links.
- N2: fix stale doc paths (crates/tty/ and docs/research/).
- N3: amend AGENTS.md §14 to accurately describe the local module's
  libc::kill unsafe blocks.
- N5: consolidate nanos_seed into tests/common/mod.rs.

Verification: cargo test (84), cargo test --all-features (107),
clippy clean (native + wasm), fmt clean, doc clean, wasm check clean.
Coverage: session.rs 79.71% -> 87.43%, total 90.74% -> 91.47%.

AGENTS.md

@@ -213,11 +213,16 @@ implementation agents.
     modules, `PascalCase` for types/traits, `SCREAMING_SNAKE_CASE` for
     constants (`STREAM_STDIN`, `TTY_OPEN_SCOPE`, `CHUNK_HEADER_LEN`).
 
-14. **No `unsafe`** — the crate has zero `unsafe` blocks. The PTY
-    bridge's `libc::kill(-pgid, sig)` and `libc::kill(pid, sig)` calls
-    are safe `libc` crate APIs (not `unsafe` blocks in this crate);
-    bounds-checked slice access via `get(..)`/`ok_or_else` is the
-    pattern. Do not introduce `unsafe` for performance.
+14. **No `unsafe`** — the crate has zero `unsafe` blocks outside the
+    `local` feature module's signal-forwarding calls. The PTY bridge's
+    `libc::kill(-pgid, sig)` and `libc::kill(pid, sig)` calls (and the
+    pipe-mode `libc::kill(pid, sig)` / `libc::kill(pid, SIGKILL)`
+    fallback) are safe `libc` crate APIs wrapped in `unsafe { ... }`
+    blocks because `libc::kill` is an `unsafe fn`; they are the
+    documented signal-forwarding path, not `unsafe` in the crate's own
+    logic. Bounds-checked slice access via `get(..)`/`ok_or_else` is
+    the pattern. Do not introduce `unsafe` for performance, and do not
+    add `unsafe` outside the `local` module's `libc::kill` calls.
 
 ## Verification Commands