feat(e2e): add interactive TTY mode with expectrl for stdin testing

Add support for interactive TTY mode in e2e test runner using expectrl
crate. This enables tests to simulate interactive stdin input through
a PTY pseudo-terminal.

Key changes:
- Modified Step enum: `stdin: Str` → `interactive: bool`
- Added run_interactive_step function using sync expectrl with spawn_blocking
- Implemented [write-stdin:...] protocol for writing to stdin
- Empty content [write-stdin:] signals EOF (sends Ctrl-D)
- Updated stdin-passthrough fixture to use new protocol
- Created echo-stdin.js script that verifies TTY mode

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: branchseer

Cargo.lock

@@ -37,6 +37,7 @@ allocator-api2 = { version = "0.2.21", default-features = false, features = ["al
 anyhow = "1.0.98"
 assert2 = "0.3.16"
 assertables = "9.8.1"
+async-compat = "0.2"
 async-trait = "0.1.89"
 base64 = "0.22.1"
 bincode = "2.0.1"
@@ -63,6 +64,7 @@ derive_more = "2.0.1"
 diff-struct = "0.5.3"
 directories = "6.0.0"
 elf = { version = "0.8.0", default-features = false }
+expectrl = "0.8.0"
 flate2 = "1.0.35"
 fspy = { path = "crates/fspy" }
 fspy_detours_sys = { path = "crates/fspy_detours_sys" }
@@ -73,6 +75,7 @@ fspy_shared = { path = "crates/fspy_shared" }
 fspy_shared_unix = { path = "crates/fspy_shared_unix" }
 fspy_test_utils = { path = "crates/fspy_test_utils" }
 futures = "0.3.31"
+futures-lite = "1.13.0"
 futures-util = "0.3.31"
 insta = "1.44.3"
 libc = "0.2.172"

Cargo.toml

@@ -22,9 +22,13 @@ vite_task = { workspace = true }
 which = { workspace = true }
 
 [dev-dependencies]
+async-compat = { workspace = true }
 copy_dir = { workspace = true }
 cow-utils = { workspace = true }
+expectrl = { workspace = true }
+futures-lite = { workspace = true }
 insta = { workspace = true, features = ["glob", "json", "redactions", "filters", "ron"] }
+nix = { workspace = true }
 regex = { workspace = true }
 serde = { workspace = true, features = ["derive", "rc"] }
 tempfile = { workspace = true }