fix(extract): give Go build-constrained twin files distinct QNs

[ilyabrykau-orca]

Part of #1911 — draft implementation-proposal: this changes what gets extracted (Go func/method QNs in build-constrained files), so per CONTRIBUTING it's opened as a draft for design review; the design is the one proposed in the issue body (the #495 Rust-cfg #-suffix pattern). Will mark ready on a maintainer design ack.

What

Build-constrained twin files (//go:build lines, GOOS/GOARCH filename suffixes) legally define the same symbols; the upsert kept ONE node per QN by the smallest-file-path rule — a 2-line stub beat the 63-line implementation and took all 25 inbound CALLS; a bpf2go GOARCH twin pair lost 39/40 nodes; coverage stayed silent.

How

• τ = compacted //go:build expression, else the official GOOS/GOARCH filename suffix (cbm_go_build_tau, computed once per file). Folded into func/method QNs (MirrorConfig#unix), mirrored on the call-scope side (go_tau_scope_qn) so body-call attribution stays exact. Types/vars stay plain — parent_class/DEFINES_METHOD joins keep working.
• Two #-aware seams make suffixed defs resolvable (both also close a latent gap for Rust cfg twins, whose QNs already carry #):
  • cbm_registry_add indexes the simple name as the part before # (identifiers cannot contain one);
  • the LSP registry's exact pkg.name lookup falls back to a sole #-suffixed variant (lookup_func_sole_tau_variant) — the dominant real-world shape is a constrained file with no in-tree twin, and its callers keep their lsp_direct/lsp_strategy_cross_file edges. Two+ variants are genuinely ambiguous without the caller's build configuration and fail closed. The pipeline's LSP-join leaf gate learns the same rule.

Scope (deliberately the issue's "minimal PR")

QN disambiguation only. τ-aware resolution (preferring the caller's own constraint, and module.name#τ same-module keys) is the declared follow-up — unconstrained callers of a genuine twin pair currently resolve as ambiguous rather than binding an arbitrary twin, which is strictly better than today's bind-everyone-to-the-stub.

Tests (reproduce-first)

• extract_go_buildtag_tau_in_func_qns: //go:build linux && amd64 → #linux&&amd64, _windows_amd64.go → #windows_amd64, _linux_test.go → #linux, types stay plain, unconstrained files carry no #. RED on main.
• pipeline_go_buildtag_twins_both_survive: RED on main (count_nodes_named == 1, expected 2); GREEN with both twins as nodes AND a sole-variant caller edge staying alive (UseFlush → FlushDisk#linux, lsp_strategy_cross_file @ 0.92).
• 854 green across extraction/pipeline/registry/lsp probes/go_lsp/parallel; full scripts/test.sh venue leg green; git clang-format --diff clean.

Field census

Pending: the validation index of the reference Go+C repo is currently blocked by a live-daemon rendezvous conflict on this machine (isolated-HOME one-shots get conflicting CBM process is active); numbers (bpf2go twin-pair node recovery 43+1 → ~86, Close 63 → 66, CALLS deltas) will follow as a PR comment once it clears. Expected shapes are in #1911's body.

#1932 tracks the family.

[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.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

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.

[ilyabrykau-orca]

CI retrigger (no-op amend): test-lsan-macos failed in test_cli installer/skill probes and test_daemon_ipc (no LeakSanitizer report anywhere in the log) — none of which this Go-extraction diff touches, and the same job is green on every sibling PR in today's waves (#1907 #1913 #1937 #1938 #1944 #1947 #1949). Same runner-environment day as #1952. No semantic changes.