feat(editor): add typing and region indentation

[owainlewis]

Tab now inserts spaces to the next four-column stop, and Enter carries leading spaces/tabs while preserving LF or CRLF style.
Tab and Shift-Tab indent or outdent selected lines as one undo step, preserving selection direction and excluding an end at the next line's start.
Shift-Tab without a region removes up to four columns of leading indentation.
Named indent and outdent commands share these behaviors.

Single-line operations change only the whitespace prefix, so a long line does not create full-line undo copies.
Closes #164.

Verification:

• 352 unit tests pass, covering Unicode display width/graphemes, existing tabs, newline style and split points, forward/reverse regions, blank lines, undo/redo, and bounded single-line history payload.
• Eight terminal integration tests pass locally; the two redirected-input cases remain restricted by the local sandbox and require the existing GitHub checks.
• Formatting, all-target Clippy, and release build pass.
• Release PTY flow: nested Rust typing, region indent/outdent, undo/redo across saves, clean exit, restored terminal settings, and usable shell.
• Independent review: Approve.

[greptile-apps]

Greptile Summary

This PR adds indentation-aware typing and region editing while preserving editor history, selection direction, grapheme boundaries, and existing newline conventions.

• Tab inserts spaces to the next four-column display stop.
• Enter carries the available leading indentation and preserves LF or CRLF style.
• Tab and Shift-Tab indent or outdent selected lines as one undoable edit.
• Named indentation commands share keyboard behavior.
• Focused tests cover Unicode, mixed whitespace, region boundaries, line endings, history size, and undo/redo.

Confidence Score: 5/5

The PR appears safe to merge; no actionable correctness, security, or repository-rule issues remain.

The command paths, prefix-only text replacement, point and mark mapping, newline fallback behavior, and mixed-whitespace handling are internally consistent and supported by focused tests.

Important Files Changed

Filename	Overview
src/commands.rs	Implements newline indentation, tab-stop insertion, prefix-only outdent, region replacement, endpoint mapping, and focused behavioral tests.
src/app.rs	Routes keyboard and named indentation commands through shared region-aware behavior while preserving selection direction.
src/buffer.rs	Adds bounded indentation and newline-style queries and exposes point-aware replacement for compact undo records.
src/input.rs	Adds constrained conversion of terminal BackTab events into the editor’s Shift-Tab key.
src/keymap.rs	Maps Tab and BackTab to the new indentation commands.
src/command_registry.rs	Registers named indent and outdent commands and updates newline command documentation.
README.md	Documents tab stops, carried indentation, line-ending preservation, region behavior, and named commands.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Keyboard or named command] --> B{Indent or Outdent?}
  B -->|No active region| C[Dispatch single-line command]
  C --> D{Indent?}
  D -->|Yes| E[Insert spaces to next tab stop]
  D -->|No| F[Remove up to four leading columns]
  B -->|Active region| G[Normalize selected line range]
  G --> H[Exclude endpoint at next line start]
  H --> I[Replace changed prefixes as one edit]
  I --> J[Map point and mark through edit]
  J --> K[Preserve selection direction]

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