fix(extraction): preload the Objective-C grammar for C-family headers (#1628)

[maxmilian]

Fixes #1628.

A .h file is classified from its path alone as C, so the preload set handed to the parse workers contains c — plus cpp, added explicitly because a header might turn out to be C++. Parse-time detection then reads the source and can also return objc (detectLanguage's .h branch checks looksLikeCpp and looksLikeObjc), and no worker holds that grammar. The file fails outright with Failed to get parser for language: objc and nothing in it reaches the index.

That is why the failure looked arbitrary: any .m file in the project pulls objc into the set by path, so the very same header indexes fine. @Juddd's control experiment — adding a one-line grammar_seed.m and watching both files index — pins it exactly.

So objc is preloaded alongside cpp whenever c is present: both are readings that content-aware .h detection can produce, and both have to be available before the workers start.

Verified against the reporter's reproduction

A directory containing only:

@interface CGRepro
- (void)run;
@end

Before (codegraph init --yes on current main):

1 parser initialization failures
# .codegraph/errors.log
repro.h: Failed to get parser for language: objc

After:

Indexed 1 files
2 nodes, 1 edges in 315ms

One refactor, and why it's in scope

The three lines that build this set existed twice — once for the full index, once for the changed-file reindex — and fixing only the copy the issue points at leaves the other one carrying the same bug for incremental runs. Both now call preloadLanguagesForFiles(), which also makes the rule unit-testable instead of buried inside a 200-line method.

Tests

__tests__/preload-languages.test.ts, four cases: both ambiguous readings of a .h are covered; a project with no C-family header gets neither grammar added; a language the files already need isn't duplicated; extension overrides still drive the base set.

Confirmed red by reverting just the objc half of the change (expected [ 'c', 'cpp' ] to include 'objc'), so the test pins this fix rather than restating the implementation.

Full suite: 179 files / 3056 tests passing (3052 on main plus these four). tsc --noEmit clean.

Reported and diagnosed by @Juddd.