Fix Windows backspace and initial prompt

[l1a]

What & why

Two fixes for connecting from a Windows etr client to a Unix etrs host. Both were invisible over plain ssh from the same Windows machine, which is why they looked like etr-specific regressions.

• Backspace behaved like a delete / didn't erase. The Windows console delivers legacy key codes to raw byte reads (Backspace as 0x08), whereas a Unix PTY expects the xterm convention 0x7f (DEL) to match the default stty erase. etr forwarded the raw 0x08 verbatim, so the remote never saw the erase char it expected. The client now switches the console into virtual-terminal mode after enabling raw mode — ENABLE_VIRTUAL_TERMINAL_INPUT on stdin (keys become the same VT byte sequences a real terminal emits: Backspace → DEL, arrows → escape sequences) and ENABLE_VIRTUAL_TERMINAL_PROCESSING on stdout (renders the remote's ANSI). No-op on Unix. New Windows-only dependency: windows-sys (Console).

• Blank screen until the first Enter. etrs spawned the shell and started reading its PTY at session start, before any client connected. The shell's initial prompt could be produced in the window between the server snapshotting replay data and installing the live PTY channel — so on the first connection it was recorded to history but neither replayed nor sent, and pressing Enter forced a fresh prompt. The PTY reader task is now started lazily on the first connection, after the client's PTY channel is live (mirrors how SSH emits shell output only once the channel exists). Replay-on-reconnect is unchanged — the reader persists across reconnects and picks up each new outbound channel.

Also bumped two transitive deps to clear RustSec advisories published while this branch was in flight (neither introduced here): crossbeam-epoch 0.9.18→0.9.20 (RUSTSEC-2026-0204, dev-only via criterion, not in the shipped binaries) and anyhow 1.0.102→1.0.103 (RUSTSEC-2026-0190 unsoundness).

Version bumped 0.6.2 → 0.6.3; NOTES.md, man pages, and the wiki Troubleshooting page updated.

Test plan

• cargo fmt --check, cargo clippy --all-targets -D warnings, cargo test (110) — green on Windows (x86_64-pc-windows-msvc) and Linux (WSL Fedora; the #[cfg(unix)] etrs path compiles, lints, and tests clean).
• CI matrix green on ubuntu x86/arm, macOS, Windows, plus Security Audit.
• Manual verification for a reviewer (needs a Windows client + a Unix host, which CI cannot exercise):
  1. Build etr on Windows; deploy etrs 0.6.3 to a Linux host.
  2. etr user@host from a native Windows console (PowerShell/Windows Terminal).
  3. Confirm the shell prompt appears immediately, without pressing Enter.
  4. Type some text and press Backspace — it should erase left, and stty -a should show erase = ^?.
  5. Drop the network and confirm reconnect still replays session output.

🤖 Generated with Claude Code

[l1a]

Superseded by #55, which keeps only the confirmed-working Windows Backspace fix plus the dependency-advisory bumps. The server-side deferred-PTY-reader change and the crossterm-events input rewrite from this branch were experiments targeting the 'first line not echoed until Enter' bug; that bug is still unresolved and is now tracked in #54. Closing in favor of #55.