refactor(env): introduce VarName enum to unify nameref resolution API

[lu-zero]

Summary

This is a stacked PR on top of the nameref-expanded branch. It refactors the environment API to use a VarName enum that encodes nameref resolution strategy in the type system, eliminating the dual-method pattern.

Problem

The nameref implementation added parallel method pairs to ShellEnvironment:

• update_or_add() / update_or_add_bypassing_nameref()
• unset() / unset_bypassing_nameref()
• update_or_add_array_element() / update_or_add_array_element_raw()
• Plus IntoVarLookup/IntoVarLookupMut traits + DirectVarLookup/DirectVarLookupMut builder types

Every caller had to pick the right variant, and getting it wrong produced subtle bash-incompatibility bugs.

Solution

Introduced a VarName enum with three variants:

enum VarName {
    Auto(String),                              // follow nameref chains (default for &str)
    Resolved { base: String, subscript: Option<String> },  // already resolved
    Direct(String),                            // bypass nameref resolution
}

• &str / String convert to VarName::Auto by default (backward compatible)
• ResolvedName converts to VarName::Resolved via From impl
• VarName::direct("name") for the bypass case

Eliminated methods:

• Dual update_or_add / update_or_add_bypassing_nameref → single update_or_add(impl Into<VarName>)
• Dual unset / unset_bypassing_nameref → single unset(impl Into<VarName>)
• Dual update_or_add_array_element / _raw → single method
• 4 trait types + 4 builder types → 2 builder types (VarLookup / VarLookupMut)

Module split: env.rs (2128 lines) → env/mod.rs + env/names.rs

Lookup builder changes

Before	After
env.lookup("name").get()	Same (auto-resolve)
env.lookup("name").bypassing_nameref().get()	env.lookup("name").get_direct()
env.lookup(&resolved).get()	env.lookup(&resolved).get() (returns ResolvedVarRef) or .get_direct() for raw
env.lookup("name").bypassing_nameref().in_scope(p).get()	env.lookup(VarName::direct("name")).in_scope(p).get_direct()

Test results

• Before: 99 brush-core tests passing
• After: 108 brush-core tests passing (+9 new VarName-specific tests)
• All workspace tests pass, clippy clean, fmt clean

Files changed

• brush-core/src/env/names.rs — new: VarName, ResolvedName, name validation functions
• brush-core/src/env/mod.rs — refactored ShellEnvironment methods
• brush-core/src/expansion.rs, interp.rs, extendedtests.rs, shell.rs — adapted callers
• brush-builtins/src/{declare,export,unset,mapfile,read}.rs — adapted callers

Future work (Phase 2)

The VarName enum uses String fields now. When we add string interning (e.g., lasso), we can introduce a Name newtype and swap the fields to interned keys, making VarName Copy.

[reubeno]

Thanks for helping with these changes! I've skimmed through and I'm going to start merging some of it into my working branch -- but there's a few pieces I'd like to think more about first (on the read-only lookup side).