Communicate rendering credit usage in the TUI

[pedjaradenkovic]

Implements Codeplain-ai/next-microsoft#13 — communicate rendering credit usage in the TUI.

Note: the tracking issue lives in next-microsoft, but the TUI code is in this repo.

What it does

Adds a credit-usage line beneath the render status — functionalities N used credits N render time T — matching the Figma design:

• During render: live line under rendering in progress ...; render time ticks each second, counts update as functionalities complete.
• On success: frozen totals under ✓ rendering completed! / generated code folder: … (wording synced to the console summary).
• On failure: usage line placed below the error message.

used credits == functionalities — 1 credit is charged per functional requirement.

Key design points

• usage_summary.py (new, root): single source of truth for the line's wording/markup, shared by the console summary (cli_output) and the TUI. Kept at root so tui stays independent of cli_output/the renderer.
• One elapsed-time accessor, RunState.get_live_render_time(): the render time is computed in exactly one place and reused by the log timestamps (previously duplicated in ElapsedTimeFormatter and LoggingHandler), the TUI usage line, and the post-render summary. add_to_render_time resets the segment start after banking so the value isn't re-counted once a render completes.
• The TUI owns only the refresh cadence: it reads get_live_render_time() and freezes the line while paused so it's never sampled inside the render machine's pause loop. On every terminal path it captures the live value onto run_state, so the TUI line and the console summary always agree.

Bug fixes included

• format_duration_hms mangled whole-second inputs (10 → "1s") — this also affected the existing console summary and log timestamps.
• Render time showed 0 on failure and on cancellation (the render machine doesn't finalize its time when an exception/cancel unwinds out of the render loop) — now the shared live value supplies the real elapsed time.

Tests

New: tests/test_usage_summary.py, tests/test_plain2code_utils.py, tests/test_plain2code_state.py (elapsed-time accounting), and tests/test_tui_usage.py (headless Textual run_test covering the live line, success/failure/cancel states, and pause-freeze).

black / isort / flake8 / mypy all clean. One pre-existing, unrelated failure remains on main (tests/test_plainfileparser.py::test_code_variables).

Open question for review

The generated code folder: path is currently shown absolute (pre-existing behavior, unchanged here); the mockup shows it relative. Left as-is per discussion — to be decided separately.