Support aliased imports for external packages

[heartwilltell]

Phase 5: Compiler Integration — RFC #218

Implement aliased imports (use mux "github.com/user/router") so users can rename packages on import. The alias becomes the local symbol name instead of the last path segment.

Closes #221.

Example

use mux "github.com/user/router"    // use as: mux.Handle(...)
use "github.com/user/repo/auth"     // use as: auth.Verify(...)

Acceptance Criteria

• Parser accepts use <ident> <string> syntax
• Alias identifier becomes the package symbol name
• Original path is still used for resolution
• Non-aliased imports continue to work unchanged
• Error if alias conflicts with another import name
• Tests for aliased and non-aliased imports

Files

• Modified: src/parser.zig (new import syntax)
• Modified: src/ast.zig (potentially new node variant)
• Modified: src/resolve.zig

Dependencies

• Extend import resolution to recognize external module paths #312 (external import resolution)

Independent of multi-file compilation (#314-316).

Part of the Package Manager RFC.