fix(run): raw-mode-safe CRLF for launch-path status banners

[evgeny-boger]

Problem

Status lines printed during launch staircase under raw mode — the next line is shoved to the right:

==> A newer image is available in the registry (cached 273bc18ea563, registry 75fd20fd31c2)
                                                                                           ==> Run `agent-vm pull` to fetch it. Continuing with the cached image.

Root cause

Several launch-path messages are emitted from background tasks that can run after the interactive session has switched the terminal to raw mode (crossterm::enable_raw_mode in the exec path):

• the update-available banner (notify_if_update_available), and
• the --auto-publish "watching" line and per-port event lines.

In raw mode the terminal's ONLCR post-processing is off, so the bare \n from eprintln! is a line feed with no carriage return: the cursor drops a row but keeps its column.

Fix

Introduce one shared helper, status_line_ending(is_tty), that returns:

• \r\n on a TTY — renders flush-left whether or not raw mode is active (harmless in cooked mode);
• plain \n for redirected stderr — no stray carriage returns in logs/pipes.

Route every racing launch-path status line through it:

• update_banner (update-available banner), and
• the three --auto-publish messages (eprint!("…{nl}"); the per-event task samples is_terminal() once before its loop — a fd's TTY-ness is stable for the process).

Also converts the now-single-arm match in notify_if_update_available to if let (silences clippy::single_match).

Tests

• status_line_ending_is_crlf_on_tty_and_lf_otherwise
• update_banner_uses_crlf_on_tty_so_second_line_doesnt_staircase
• update_banner_uses_plain_lf_off_tty
• cargo build, cargo test -p agent-vm (131/131 with --test-threads=1; one unrelated pre-existing flake races on a process-wide env var under parallel runs), and cargo clippy -p agent-vm all pass — zero new clippy warnings (net −1).

End-to-end (real binary)

Ran the actual compiled agent-vm binary inside a raw-mode PTY (slave in cfmakeraw, i.e. the post-enable_raw_mode state), with a planted stale pulled-digests marker so check_for_update fires a live ghcr.io probe and hits UpdateAvailable:

• Instrumented is_terminal() at banner time → true (stderr is the PTY).
• Update-banner bytes on the wire (raw mode, no ONLCR translation): …)\r\n==> Run … cached image.\r\n — real CRLF, flush-left.

This exercises status_line_ending(stderr().is_terminal()) in the real binary. The --auto-publish lines call the identical helper but could not be driven end-to-end here: they print only after a successful Sandbox::create, and this nested dev box can't boot the guest microVM (entering VM → Vmm is stopping — nested KVM is unavailable). They're covered by the shared helper's real-binary proof + the unit test + code review.

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