fix: use ns::Name as X binds the alias X (closes #43)

[Kinflou]

Completes #43. #44 made a plain use ns::Name bind bare Name; this adds aliases.

IR change

Import(String, (usize, usize))
// ->
Import(String, Option<String>, (usize, usize))   // (resolved path, local alias, span)

The alias is the X in use ns::Name as X; None for a plain use.

Behaviour

declaration	binds
use types::User	types::User (qualified) and bare User
use types::User as Account	types::User (qualified) and Account — not bare User
use types::* / use types::{A, B}	per-symbol qualified + bare (alias meaningless, ignored)

Rust-style.

How

• resolve_use_declaration takes the alias and attaches it to the single-symbol / whole-namespace Import unit.
• SymbolTable gains a private bare_imports: HashSet — the alias, or the trailing segment of a plain use. It's kept out of symbols so it never trips duplicate detection and a real local declaration always shadows it. is_imported_bare checks that set (replacing the fix: use ns::Name binds the bare name Name in validation (#43) #44 scan).

Churn

~10 mechanical Import(x, y) → Import(x, None, y) construction sites in incremental.rs; Import(p, _) → Import(p, _, _) in 3 test files. CAS/serde handle the extra field transparently.

Tests

imports.rs: test_use_binds_the_bare_name, test_use_as_binds_the_alias (also asserts the Import unit carries alias = Some("Account")), test_use_as_does_not_bind_the_original_bare_name. from_sources.rs::cross_schema_use_resolves_across_the_added_schemas now covers bare / qualified / alias. Full suite green.