v0.4.0 — inline fzf-style mode + manage rebind

sshs v0.4.0 introduces an inline (fzf-style) host browser as the
default command. Type to filter, Enter to ssh, and you're back to
your shell — no alternate-screen takeover, no UI re-entry after ssh.

The full v0.3 management TUI is one flag away: sshc -m (or --manage).

Added

• Inline mode (default sshc). Opens a ~15-line viewport BELOW
  the shell prompt via ratatui's Viewport::Inline. Type to filter
  immediately (fzf muscle memory), ↑/↓ or j/k to navigate,
  Enter to ssh, Esc/Ctrl+C to cancel, r to reconnect to the
  last alias. Viewport area is cleared on exit so the shell sees no
  residue. Terminal below 12 rows falls back to manage mode with a
  one-line stderr notice.
• Manage mode (-m / --manage). The v0.3 alternate-screen
  TUI, retained behind a flag. Default behaviour changed in v0.4 —
  intentional breaking change for a single-user tool.
• Manage-mode key rebind (BRIEF §5):
  • Enter opens the modify form for sshs.conf-managed hosts;
    falls through to $EDITOR for external-source hosts.
  • s — ssh connect (was Enter in v0.3).
  • m — unbound (merged into Enter).
• ScreenMode { Alternate, Inline(u16) } on TerminalGuard.
  RAW_ACTIVE and ALT_ACTIVE tracked separately so the panic hook
  doesn't emit LeaveAlternateScreen in inline mode (would
  corrupt a normal-mode terminal).
• InlineApp — lean read-only host browser (no probes, no
  modals, no forms, no storage writes).
• tui::inline_runtime — single-shot event loop +
  handle_connect_inline that tears down the viewport before ssh
  spawn and exits the process with an SshResult-derived ExitCode.
• src/run.rs — inline() / manage() dispatch helpers.
  Keeps main.rs thin (41 LOC, R-G4).
• R-G9 boundary gate enforces inline_app's read-only scope.
• examples/inline_prototype.rs — standalone ratatui Viewport
  smoke test, useful for terminal-compat triage.

Changed

• main() returns ExitCode (Termination). ssh exit codes
  propagate to the parent shell from inline mode (Quit → SUCCESS;
  Connect/Reconnect → low byte of the ssh code;
  Crashed/UnknownTermination → FAILURE).
• TerminalGuard's acquire takes a ScreenMode. Existing v0.3
  callers pass ScreenMode::Alternate for unchanged behaviour.

Tests

162 (v0.3.0) → 190 (v0.4.0). New: 13 InlineApp unit tests + 5
inline integration tests + ScreenMode equality + 3 manage-rebind
tests (s connects, Enter on external, Enter on managed, m
unbound). All R-G1..R-G9 grep gates clean.

Compatibility

• v0.3 users running sshc land in inline mode on first launch.
  Selection + Enter ssh-connect work as expected.
• For the v0.3 behaviour, use sshc -m.
• ~/.ssh/config, ~/.ssh/config.d/sshs.conf, state.toml
  unchanged. First-run setup flow (Include injection) runs in
  manage mode only.

See CHANGELOG.md for the full diff vs v0.3.0.