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).

examples/1176-diagnostics-import-parse-error-location.sx

@@ -0,0 +1,13 @@
+// A parse error in an IMPORTED file must be located in THAT file, not the
+// importer. Regression: `import_sources` was wired to the diagnostics only
+// AFTER import resolution finished, so a parse error raised MID-resolution
+// (which aborts before that wiring) could not resolve the imported file's
+// source — the caret fell back to the root file and landed on an unrelated
+// line. The fix wires `import_sources` before resolving and pins the
+// diagnostic's `source_file` + offset to the imported file.
+//
+// The companion's error sits several lines down (after comments) so a caret
+// mislocated against THIS importer would be unmistakable.
+#import "1176-diagnostics-import-parse-error-location/broken.sx";
+
+main :: () {}

examples/1176-diagnostics-import-parse-error-location/broken.sx

@@ -0,0 +1,6 @@
+// Deliberately broken: exercises import parse-error LOCATION reporting.
+// These leading comment lines push the parse error down so its line number
+// differs from the importer's — a mislocated caret would point here-or-wrong
+// instead of at the real offending token below.
+//
+broken :: 1 2;

examples/expected/1176-diagnostics-import-parse-error-location.exit

@@ -0,0 +1 @@
+1

examples/expected/1176-diagnostics-import-parse-error-location.stderr

@@ -0,0 +1,5 @@
+error: parse error in 'examples/1176-diagnostics-import-parse-error-location/broken.sx': expected ';'
+  --> examples/1176-diagnostics-import-parse-error-location/broken.sx:6:13
+   |
+ 6 | broken :: 1 2;
+   |             ^

examples/expected/1176-diagnostics-import-parse-error-location.stdout

@@ -0,0 +1 @@
+