fix(diagnostics): locate import parse errors in the imported file

A parse error raised while resolving an `#import` was rendered against the
ROOT file's source — the caret landed on an unrelated line (often a comment)
even though the message named the correct imported file.

Two compounding causes:
- core.zig wired `diagnostics.import_sources` only AFTER import resolution
  returned, but a parse error aborts mid-resolution (before that wiring), so
  the renderer had no imported sources and fell back to the root file. Wire it
  (and seed the main-file source) BEFORE resolving.
- imports.zig emitted the diagnostic at the importer's `#import` span instead
  of the parser's actual error offset inside the imported file, and didn't pin
  the diagnostic's source_file to that file.

parser.zig now records `err_end` alongside `err_offset` for a proper caret
width. New `DiagnosticList.addFmtInFile` renders against an explicit source
file; imports.zig uses it with `importErrSpan(&p)`.

Regression test: examples/1176-diagnostics-import-parse-error-location
(importer + deliberately-broken companion; caret must land in the companion).

src/errors.zig

@@ -179,6 +179,18 @@ pub const DiagnosticList = struct {
         _ = self.addFmtId(level, span, fmt, args);
     }
 
+    /// Like `addFmt`, but renders against an EXPLICIT source file (resolved via
+    /// `import_sources`) instead of the ambient `current_source_file`. Used to
+    /// pin a diagnostic whose `span` is an offset into a NON-current file — e.g.
+    /// a parse error raised while resolving an `#import`, where the span belongs
+    /// to the imported file, not the importer.
+    pub fn addFmtInFile(self: *DiagnosticList, level: Level, source_file: []const u8, span: ?Span, comptime fmt: []const u8, args: anytype) void {
+        const saved = self.current_source_file;
+        self.current_source_file = source_file;
+        defer self.current_source_file = saved;
+        _ = self.addFmtId(level, span, fmt, args);
+    }
+
     pub fn addFmtId(self: *DiagnosticList, level: Level, span: ?Span, comptime fmt: []const u8, args: anytype) usize {
         const message = std.fmt.allocPrint(self.allocator, fmt, args) catch "diagnostic format error";
         return self.addId(level, message, span);