fix(extract): drop cgo's import "C" pseudo-package

[ilyabrykau-orca]

Fixes #1926

Problem

import "C" is not an import. "C" is reserved by go/build; no package with that
import path can exist. The clause only tells the Go toolchain to compile the
immediately preceding comment block as C.

The Go import parser treated it as an ordinary import spec, so it reached
cbm_pipeline_resolve_import_node(), fell through Strategy 1 (no module resolves) and
Strategy 2 (no namespace map), and landed in the Strategy 3 symbol-name fallback
(src/pipeline/pass_pkgmap.c:1904-1998). That looks up every project node named C and
returns the targetable candidate with the lexicographically smallest qualified name.

Measured on a real Go+C repository (~1150 Go files): all 27 files carrying
import "C" pointed their IMPORTS edge at the same unrelated C member of a test
helper — 27 false edges, and 27 files whose import map named a package that cannot
exist. Which symbol gets hijacked is decided by a lexicographic tie-break, so it moves
as the repo grows.

Change

internal/cbm/extract_imports.c, parse_go_import_spec() — skip the spec when the
import path is exactly "C", before the import is pushed. Go-specific knowledge belongs
in the Go import parser rather than in the language-agnostic resolver, and dropping it at
extraction also keeps the pseudo-package out of the file's import map used by call
resolution.

Effect

Nodes are unchanged and the file's real imports are untouched; only the false edge
disappears.

	nodes	edges	IMPORTS with local_name = 'C'
3-file fixture, before	14	22	1
3-file fixture, after	14	21	0
real repo, before	—	—	27
real repo, after	—	—	0

Test

tests/test_extraction.c — go_cgo_pseudo_import_dropped.

Reproduce-first: RED without the extractor change —

go_cgo_pseudo_import_dropped   FAIL tests/test_extraction.c:2881:
  ASSERT(strcmp(r->imports.items[i].module_path, "C") != 0)
  318 passed, 1 failed

GREEN with it — 319 passed.

The test also asserts the same file's real fmt import survives, so the fix cannot
regress into dropping the whole import block.

Checks

• scripts/test.sh --suites extraction — 319 passed, 0 failed.
• clang-format clean on both edited hunks.
• scripts/lint.sh could not complete locally: clang-tidy and cppcheck are not
  installed on this machine. Relying on CI for those two legs.

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
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If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.