Fix Linux terminal input loss during resize

[swerner]

Linux terminal input can stall after a resize when Enter is already waiting. Crossterm 0.29's default Mio reader returns the first event from a readiness batch and can discard the other edge-triggered notification. The first-run key prompt then waits until the terminal test times out. The internal dependency update run tracked by #39 and the latest nightly both show this failure.

Enable Crossterm's use-dev-tty polling backend only on Linux. Add a PTY regression that queues resize and Enter before the reader resumes and requires both events to arrive. macOS keeps its existing backend: applying this feature there changed terminal-mode restoration in the earlier experiment.

Add filedescriptor 0.8.3, the polling backend's descriptor wrapper. Its thiserror / thiserror-impl 1.0.69 versions already exist in Fabro's lockfile. Existing package versions and git revisions are unchanged.

Fixes #39.

Validation:

• mise run check passed on macOS: 1,683 tests passed, one skipped; formatting, Clippy, doctests, documentation, library feature checks, workflow audit, and whitespace checks passed.
• The new Linux regression fails on the default backend with enter=false resize=true.
• Earlier isolated backend testing delivered both pending events in 100/100 Linux trials, and the original first-run reproduction passed 100/100 trials with the Linux feature enabled. The Linux and macOS TUI suites also passed in that experiment.
• The full routine CI gate passed on macOS, Linux x86_64, and Linux arm64 at b15f47e, including the new Linux regression and the original first-run test. The current implementation and dependency manifests are unchanged from that tested commit. Whitespace checks passed locally, and PR CI will run again on the latest commit.