Fix --wrap-width truncating code blocks + pin Rust toolchain

[jvanderberg]

Summary

Closes #55. Three related changes plus a CI hygiene fix.

1. Don't let --wrap-width truncate code blocks (src/document/parser.rs, src/app/model.rs, src/document/mod.rs, src/app/event_loop.rs)

wrap_width was used both to word-wrap prose AND to clamp the maximum width of non-wrapping elements like code blocks. Setting --wrap-width 80 on a wider terminal silently truncated code lines >76 chars, which broke copy/paste of those lines.

Split the parser's two width inputs:

• wrap_width — controls prose word-wrap (unchanged behavior)
• max_width — upper bound for elements that don't wrap (code blocks, tables); equals the available terminal area regardless of --wrap-width

New public API: Document::parse_with_widths, parse_with_all_options_widths, prepare_document_from_bytes_with_widths. Old API preserved (forwards width as both).

2. Copy the raw code block source, not the rendered line (src/app/model.rs, src/document/types.rs)

When a line is still wider than the terminal itself, it has to clip visually. The previous selection/copy path read from the rendered text, so the trailing characters were silently dropped on paste — the original complaint in #55. Selection now looks up Document::code_block_raw_line for any code body line and copies that verbatim.

3. Maximize visible content when truncation is unavoidable (src/document/parser.rs)

Even after fix #1, lines longer than the terminal need to clip. The old clamp at max_width - 4 left the frame at max_width + 3, so the terminal lopped off the right │, the space before it, and one padding space — visible result had two cols of trailing whitespace before the (missing) border. Clip to max_width - 2 instead so visible content extends to the screen edge. The missing right │ remains the truncation indicator.

4. Pin the Rust toolchain (rust-toolchain.toml, .github/workflows/*.yml)

Discovered while running pre-push checks for this PR: CI used dtolnay/rust-toolchain@stable, so any new stable release that added clippy lints could break CI without anyone changing code (this happened to PR #56). Pin to 1.95.0 across both ci.yml and release.yml and add rust-toolchain.toml so local rustup matches. Bonus: fixed 9 clippy lints from Rust 1.95 in pre-existing code (sort_by_key, collapsible_match, useless_conversion, unnecessary_trailing_comma).

Test plan

• cargo fmt --check
• cargo clippy -- -D warnings (Rust 1.95.0)
• cargo test — 626 tests pass, 5 new (select_poll_ms x3 + parse_with_widths x3 + wrap_width_does_not_truncate_code_blocks + selected_text_returns_raw_code_line_even_when_rendering_truncates)
• Manual verification with the yt-dlp curl … --import case from --wrap-width truncates code blocks without an indication #55: code block renders at full width on a wide terminal, paste contains the trailing t
• Manual verification with an over-terminal-width line: content reaches the screen edge, right │ is the missing indicator, paste still returns the original source