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/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 @@
+