helix-mode: introduce HelixRange selection type (no wiring)

[kronberger-droid]

Summary

Introduces HelixRange, a anchor/head selection range type similar to helix-core, along with a Direction enum. The type has no callers yet and is gated behind #![allow(dead_code)] until future PRs.

Motivation

Part of #960, and follow-up to #1039.

Reedline already has selection state via Editor::selection_anchor plus the insertion point. That model fits vi's visual mode but doesn't carry two things helix needs:

• Direction: anchor and head are named, not just a min/max pair.
• Persistence: in helix, every motion produces a selection that survives until the next motion or operator clears it. selection_anchor exists only when the user has actively started a selection in vi.

Rather than retrofit selection_anchor to carry direction, this PR adds a helix-local type that encodes both properties.

Why direction matters

Behaviors that depend on this:

1. Block cursor placement (one grapheme inside the head edge)
2. Selection flip — swap the active end without losing the range
3. Extending motions know which edge to grow when the head moves

What's included

• HelixRange { anchor, head } with gap-indexing semantics (left-inclusive, right-exclusive, regardless of anchor/head ordering)
• Pure-integer methods on HelixRange
• Direction::{Forward, Backward} enum
• 32 unit tests

What's deliberately deferred

• Grapheme-aware methods: translate between byte positions and graphemes, and place the visible cursor on the correct side of the selection (future PR).
• Multi-range Selection: reedline is single line.

Adaptations from helix-core

helix-core	reedline	Why
RopeSlice	&str (future PRs)	reedline uses String, not Rope
char indices	byte offsets	reedline's established convention

Tests

cargo test --features helix --lib edit_mode::helix::range

All 32 tests pass.

The module is pub(super) and gated behind the helix feature flag, thus no behavior change possible.

[fdncred]

This is a pretty small changeset with many tests (thanks). It looks good to me but I'd like to get @schlich's input on it before we land it. Thanks!

[schlich]

I'll take a look either tonight or this weekend!

[fdncred]

@schlich Have you had time to review yet? Sorry to nag.