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).
d6a9c4f0c4