Extract virtual terminal methods into runtime submodule

[ryanoneill]

Summary

• Extracts all CaptureBackend-specific impl blocks from src/app/runtime/mod.rs into a new src/app/runtime/virtual_terminal.rs submodule
• Reduces mod.rs from 1099 lines to 838 lines, bringing it under the 1000-line project limit
• No public API changes -- all existing types and methods remain accessible

Details

The extracted virtual_terminal.rs (281 lines) contains:

• Virtual terminal constructors (virtual_terminal, virtual_terminal_with_config, virtual_terminal_with_state, virtual_terminal_with_state_and_config)
• Event injection (send)
• Display inspection (display, display_ansi)
• Cell query convenience methods (cell_at, contains_text, find_text)

Test plan

• cargo clippy --all-features -- -D warnings passes with zero warnings
• cargo test --all-features passes all 787 tests (including doc tests)
• mod.rs is 838 lines (under 1000-line limit)
• virtual_terminal.rs is 281 lines (well under limit)

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.08%. Comparing base (3bddb37) to head (ad9c375).

Files with missing lines	Patch %	Lines
src/app/runtime/virtual_terminal.rs	96.42%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #211   +/-   ##
=======================================
  Coverage   92.08%   92.08%           
=======================================
  Files         144      145    +1     
  Lines       12766    12766           
=======================================
  Hits        11756    11756           
  Misses       1010     1010           

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