ltk_ritobin: Extend Visitor API surface

[Crauzer]

We should extend the Visitor API to make it easily consumable downstream.

Proposal

pub struct VisitCtx<'a> {
    pub cst: &'a Cst,
    text: &'a str,
    ancestors: &'a [NodeId],
}

impl VisitCtx<'_> {
    pub fn text(&self, span: Span) -> &str;
    pub fn ancestors(&self) -> &[NodeId];
    pub fn parent(&self) -> Option<NodeId>;
    pub fn depth(&self) -> usize;
}

// Visitor 
fn enter_tree(&mut self, ctx: &VisitCtx, node: &Node, id: NodeId) -> Visit;
fn exit_tree(&mut self, ctx: &VisitCtx, node: &Node, id: NodeId) -> Visit;
fn visit_token(&mut self, ctx: &VisitCtx, token: &Token, id: TokenId, parent: NodeId) -> Visit;

// Cst
pub fn walk_range<V: Visitor>(&self, range: Span, text: &str, visitor: &mut V)

Visit semantics

#188 landed the contract at the CST layer: exit_tree runs exactly once for every entered node while a Stop unwinds, Skip is scoped per callback, and Cst::walk returns WalkOutcome. Two things follow for this issue:

• The AST visitor from feat!(ltk_ritobin): AST #185 predates fix!(ltk_ritobin): half-open spans and balanced visitor unwinding #188 and still has the old behavior: a Stop from children skips exit on every open ancestor, and its walk_all has a Visit::Skip => break arm that skips remaining siblings undocumented. It should adopt the fix!(ltk_ritobin): half-open spans and balanced visitor unwinding #188 contract, WalkOutcome included, so downstream only has one model to hold.
• The methods proposed above inherit the fix!(ltk_ritobin): half-open spans and balanced visitor unwinding #188 semantics as written; the new tests from fix!(ltk_ritobin): half-open spans and balanced visitor unwinding #188 should be extended to cover them.

AST layer

The scope stack use case exists identically one layer up, and the AST Visitor from #185 has no ctx at all. Once AST nodes carry their originating NodeId (see #176), the AST visitor can reuse the same VisitCtx shape with ancestors as CST ids resolved on demand, so the machinery is written once instead of per layer.

walk_range

• Build it on Cst::common_ancestors (ltk_ritobin: Add Locator API #176) so it starts at the deepest node containing the range instead of re-descending from the root.
• With Span::contains half-open since fix!(ltk_ritobin): half-open spans and balanced visitor unwinding #188, the inclusion rule just needs stating explicitly, probably intersects.
• Return WalkOutcome like Cst::walk does now.

Misc

• If node spans later become TokenRange derived, a ctx.text_of(id) next to ctx.text(span) keeps callers away from how spans materialize.

Related to alanpq/ritobin-lsp#57