[cold-13]

Description

Implements the invocation timeline of the interactive replay debugger (#483).

grat replay <tx-hash> -i used to print a placeholder message. It now opens a real
Ratatui debugger that replays the transaction and shows the contract invocation
stack of the replay, so nested calls can be inspected frame by frame.

How It Was Done

crates/cli/src/tui/widgets/timeline.rs — the timeline widget:

• flatten() walks the ContractInvocation tree depth first (pre-order) and returns
  TimelineRows carrying the nesting depth, the is_last sibling flag and the
  invocation itself. The order matches the tree printed by grat trace, so a timeline
  index always points at the same invocation.
• count() returns the number of frames without allocating, and is used by the key
  handling and the footer so that navigation stays allocation free.
• render() / render_with_focus() draw the frames inside a bordered panel whose
  title shows the position (Timeline (3/12)). The border is cyan while the timeline
  owns the focus and dark gray otherwise.
• Every row shows the tree connector (├─ / └─), the execution status (✔ / ✘,
  green / red), the function name and the contract id. Stellar contract ids are 56
  characters long, so long ids are elided in the middle (CBQYN5XZ…MCC5).

crates/cli/src/tui/app.rs — the debugger shell:

• The transaction is replayed with grat_core::replay::replay_transaction before the
  terminal enters raw mode, so a failing replay keeps the regular terminal output.
• A TerminalGuard restores raw mode, the alternate screen and the cursor on drop,
  which covers normal exits, I/O errors and panics.
• The screen is split into a header (transaction hash, network, ledger), the timeline
  panel and a footer (frame counter and key bindings).
• handle_key supports q / Esc / Ctrl+C to quit and ↑/↓ (or j/k) to move
  the focused frame; TuiState::scroll_offset holds the index of the focused frame
  and is clamped to the timeline bounds.

Issues Encountered (If Any)

• The repository pins the stable Rust toolchain in rust-toolchain.toml, which could
  not be installed in the development environment, so the changes were verified with
  Rust 1.91.0.
• The full workspace build pulls the whole Soroban dependency tree, which is not
  cached locally. The TUI modules were therefore compiled and tested in isolation
  against a stub of grat_core::types::trace that mirrors the real type definitions.
• The interactive debugger and the controls widget (Implement Ratatui Debugger Controls Widget #484, PR feat(tui): implement debugger controls widget and keyboard navigation #493) both live in
  crates/cli/src/tui/app.rs, so this branch may need a small rebase once feat(tui): implement debugger controls widget and keyboard navigation #493 lands.

Related Issue

Closes #483

How It Was Tested

• 15 unit tests (cargo test) cover the flattening order and sibling flags, the
  frame counter, the elision of contract ids, the indentation of nested invocations,
  the highlight of the focused frame, the clamping of an out-of-bounds index, the
  empty-timeline message, the quit keys, the navigation bounds and the rendered
  screen content. The widget tests render into a TestBackend and assert on the
  resulting buffer.
• The code was formatted with rustfmt and compiles without warnings.

Screenshots / Video (If Applicable)

The panel renders as follows (frame 3 of 12 focused):

┌ Timeline (3/12) ─────────────────────────────────────────────┐
│ ▶ ├─ ✔ transfer CBQYN5XZ…MCC5                                │
│   ├─ ✔ mint      CBQYN5XZ…MCC5                                │
│   └─ ✘ swap      CBQYN5XZ…MCC5                                │
└───────────────────────────────────────────────────────────────┘

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