feat(terminal): pin the input line to a fixed footer row in every state

[grhohertz]

What

The prompt used to render wherever the transcript happened to end, so the input line moved with every scroll, tool call, and state change. This pins it to a fixed footer row that is present in every shell state — interactive, mid-turn, tool call, worker, coordinator, goal, idle.

Row	Content
H-3	horizontal rule (separator)
H-2	INPUT row — always the prompt
H-1	status message
H	statusline

How

• FooterLayout grows sep_row + input_row; footer height 4 (6 with an escalation tray), with graceful degradation in short windows (down to just input + bar at 4 rows, then disabled entirely).
• LAST_PROMPT caches the prompt string so every repaint path — idle heartbeat, resize, post-clear restore, mid-turn status update — redraws the resting affordance on that row. This is what makes the row unconditional rather than only-while-reading.
• Row precedence: live editor while reading → mid-turn type-ahead (MIDTURN_INPUT) → cached resting prompt.
• park_input_row / unpark_input_row move the editor onto the row for a read and hand the cursor back to the body. The row sits outside DECSTBM, so the submitted line is echoed into the body on submit and scrollback stays honest.
• print_in_body routes above-the-prompt output straight into the transcript via DECSC/DECRC instead of rustyline's ExternalPrinter, which would erase-and-redraw a prompt that no longer lives in the body.
• BODY_ROW tracks the transcript position: one bounded DSR probe per park, advanced per body write, clamped into the live body on read. Worst case degrades to "write at the bottom" — never corruption.
• All helpers no-op when no footer region is installed, so off-tty, pre-enable, and short-window runs keep classic inline behaviour.

Known limitation

A typed line that wraps past the bottom row scrolls the screen, since the editor draws outside the scroll region. The next heartbeat repaint heals the footer. Documented in-code.

Verification

cargo test --no-default-features --locked
  1829 passed / 0 failed / 0 ignored   (lib + bins, 49 in terminal::)
  + 28 plugin_dispatcher, 26 plugin_memory, 6 plugin_state,
    4 pty_harness, 4 plugin_docs, 5 plugin_integration, 2 shebang — all green
cargo clippy -D warnings   clean
cargo fmt                  applied

Files: src/terminal.rs (+384/−129), src/editor.rs, src/tools.rs, CHANGELOG.md.

Branched directly off main (99a81c5) — no conflicts.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}