feat(core): split Extend into CoverLanding vs Span

[kronberger-droid]

• New SelectionExtent (CoverLanding | Span): per-mode selection-model axis, orthogonal to CaretGeometry.
• EditCommand::Extend branches on it:
  • vi → CoverLanding (put_cursor)
  • bar modes → Span (extend_span).
• Cursor::extend_span: grows a Span to gap-indexed op_end, no vi-visual widening
• Both placement strategies funnel through Editor::place (set_cursor + commit_cursor).

Why

• Splits where the caret rests (geometry) from how far a motion drags the head (extent).
• Span is Helix groundwork, since no current mode pairs Block + Span, so that reversal path is exercised by unit tests only.

Tests

• extend_span units incl. the Helix-bound block-reversal flip.
• Successive emacs Extend(w) grows the Span.

Notes

• No public API change (all pub(crate)).
• Fully behavior preserving.
• fmt + clippy --all-targets clean; full suite green.

[u-k-g]

tested on aarch64-darwin with cargo test --all-features -- --test-threads=1: 1,216 tests passed. The Helix example also builds and runs.

cargo test --all-features -- --test-threads=1; cargo run --example helix --features helix

$ cargo test --all-features -- --test-threads=1; cargo run --example helix --features helix

Finished `test` profile [unoptimized + debuginfo] target(s) in 0.06s
Running unittests src/lib.rs (target/debug/deps/reedline-f3d584acdddd1871)

running 1217 tests
<redacted due to verbosity>

test result: ok. 1216 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out; finished in 0.09s

Doc-tests reedline

running 29 tests
<redacted due to verbosity>

test result: ok. 29 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 8.89s

Compiling crossterm v0.29.0
Compiling reedline v0.48.0 (/Users/uzair/01-projects/reedline)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 2.42s
test src/menu/menu_functions.rs - menu::menu_functions::string_difference (line 207) ... ok

test result: ok. 29 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 8.89s

Compiling crossterm v0.29.0
Compiling reedline v0.48.0 (/Users/uzair/01-projects/reedline)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 2.42s
Running `target/debug/examples/helix`
Helix edit mode demo:
Default mode is insert (`:` prompt), so you can type words.
Press Esc for normal mode.
Press `i` to return to insert mode, or `a` to insert after the current selection.
Only `h`/`l` motions are currently implemented.
Abort with Ctrl-C
~/01-projects/reedline: helix test.

Aborted!