test(tt4): Tier-4 PTY end-to-end tests for the four critical flows

Wire the Tier-4 tier from ADR-0008 — the first tests that drive the actual
built binary in a real pseudo-terminal. tests/e2e_pty.rs spawns the binary
under portable-pty at a fixed 100x30, parses output with vt100, and asserts
on the rendered screen with tight (3s) fail-fast waits. Serial (one PTY at a
time) for predictable timing on the low-parallelism runner; each test gets
its own temp --data-dir so it never touches real projects.

The four flows mirror ADR-0008's Tier-4 scope:
- cold launch -> create table -> graceful quit (Ctrl-C)
- create + add column -> quit -> reopen (--resume) -> schema (incl. column)
  restored
- export -> import into a fresh project -> schema + data rebuilt
- undo after DROP TABLE, through the confirm modal (y)

Table presence is read from the Tables sidebar region (not whole-screen)
since the Output panel echoes commands; commands are paced to completion
(real-user cadence) — a command sent mid-rebuild can be misread against a
stale schema cache (issue #39).

Also two NFR perf measurements via the harness (NFR-1 startup, NFR-3 idle
RSS) against generous debug-binary bounds for gross-regression detection;
the real release figures (median 29 ms / 10 MB) are recorded in the NFR doc.

Deps (dev): portable-pty 0.9, vt100 0.16 (both current; expectrl from
ADR-0008 dropped — it bundles a conflicting PTY layer). Runs by default in
`cargo test`, so the existing Linux CI gate exercises it (advances TT5).
PTY-in-container verified (openpty+spawn works under Docker).

Full suite: 2519 passed / 0 failed / 1 ignored. clippy + fmt clean.
Relates to TT4 / ADR-0008.

Cargo.toml

@@ -59,8 +59,10 @@ zip = { version = "5.1.1", default-features = false, features = ["deflate"] }
 
 [dev-dependencies]
 insta = { version = "1.47.2", features = ["yaml"] }
+portable-pty = "0.9"
 pretty_assertions = "1.4.1"
 tempfile = "3.27.0"
+vt100 = "0.16"
 
 # Dev/test build hygiene (see CLAUDE.md "Build hygiene"). `cargo test`
 # links ~25 separate integration-test binaries, each statically