feat(go): add coverage-summary as a Go bundle coverage slice

Third language in the coverage-summary fan-out, after Elisp and Python. Same kernel: count every source file on disk that's absent from the coverage profile as 0% and weight the project number by file, so an untested file stays visible instead of being averaged away.

The script at languages/go/claude/scripts/coverage-summary.go parses a cover.out profile, maps each import-path-qualified entry back to an on-disk relative path using the module path from go.mod, and reports a file-weighted number plus the missing files. It's standard library only, so it runs anywhere via go run, and it doesn't reimplement the per-function table that go tool cover -func already prints. I proved it against a real go test -coverprofile run, not just a synthetic fixture, since the Go toolchain is installed here.

Two findings to flag. Modern go test ./... already lists every module package in the profile at 0% even when untested, so for in-module code the missing-file list is usually empty. The detection earns its keep on build-tagged files and dirs outside ./.... And this is a coverage-only slice of a Go bundle that doesn't otherwise exist yet: there's no go.md rule file, so sync-language-bundle.sh can't fingerprint it (detection keys on a bundle's own .claude/rules). The script installs via make install-lang LANG=go but won't be sync-maintained until the Go bundle gets real rules and a CLAUDE.md. Building that out is the natural companion task.

Tests are black-box: a Go test in its own throwaway module runs the script via go run against temp fixtures and checks output, so the shipped script dir stays test-free. They cover missing-file detection, all-tracked, _test.go exclusion, and the missing-report error. make test gained a go test discovery path for languages/*/tests, guarded so environments without Go skip it cleanly.

1 files changed, 6 insertions, 7 deletions

diff --git a/todo.org b/todo.org
index 9ff1175..b6f28b8 100644
--- a/todo.org
+++ b/todo.org
@@ -1156,19 +1156,18 @@ Reference (dotemacs): =scripts/coverage-summary.el=, =modules/coverage-core.el=,
 
 Origin: handoff from the .emacs.d session, 2026-05-25.
 
-** TODO [#C] Fan out coverage-summary to Go and TypeScript bundles :feature:
+** TODO [#C] Fan out coverage-summary to the TypeScript bundle :feature:
 :PROPERTIES:
 :CREATED: [2026-05-31 Sun]
 :END:
 
-The Elisp pilot proved the pattern; Python followed (both DONE above). Python confirmed the plumbing is genuinely generic — =sync-language-bundle.sh= auto-fixes any =claude/scripts/*= and inbox-drops any =coverage-makefile.txt=; =install-lang.sh= seeds the fragment; =make test= now discovers both =languages/*/tests/test-*.el= (ERT) and =languages/*/tests/test_*.py= (pytest). So Go and TS are each just: the parser script, its tests, and the fragment.
+The Elisp pilot proved the pattern; Python and Go followed (all three DONE above). The plumbing is generic: =install-lang.sh= seeds the fragment, and =make test= now discovers ERT (=test-*.el=), pytest (=test_*.py=), and =go test= (=*_test.go=) under =languages/*/tests/=. TypeScript is the last one.
 
-- Go: =go test -coverprofile=cover.out=; parse =cover.out= (simple text), or =go tool cover -func=. Note Go has no =make test= discovery path yet — add a =go test= runner for =languages/go/tests= when this lands.
-- TypeScript/JS: nyc/Istanbul =coverage-final.json= / json-summary. Needs a JS test-discovery path in =make test= too.
+- TypeScript/JS: nyc/Istanbul =coverage-final.json= / =coverage-summary.json=. Same kernel: file-weighted project number, on-disk =*.ts=/=*.js= absent from the report counted as 0%. nyc prints its own table, so the script focuses on the missing-file list and the number. Needs a vitest/jest (or =node --test=) discovery path in =make test=, mirroring the go-test block.
 
-Keep the kernel identical: file-weighted project number, source files absent from the report counted as 0%. Don't reimplement the per-file table where the built-in reporter already prints one — Go's =go tool cover -func= and nyc both do, so those scripts focus on the missing-file list and the project number.
-
-Python notes for the next person: the script parses coverage.py's =files[path].summary.{covered_lines,num_statements}= (stable since coverage 5.x), resolves report paths against the report's parent dir (= project root), recurses the source dir for =*.py=, and was proven against a synthetic report matching the documented schema — not yet against a live =coverage json= run (coverage.py wasn't installed in the rulesets env). First real adopter should sanity-check against an actual report.
+Notes for the next person, from the Python + Go runs:
+- Python: parses coverage.py's =files[path].summary.{covered_lines,num_statements}= (stable since coverage 5.x), resolves report paths against the report's parent dir. Proven against a synthetic report, not a live =coverage json= run (coverage.py wasn't installed). Sanity-check against a real one.
+- Go: =languages/go/= is a coverage-only slice with no rule file, so =sync-language-bundle.sh= can't fingerprint it (detection keys on a bundle's own =.claude/rules/*.md=). The script is delivered by =make install-lang LANG=go= but is not sync-maintained until the Go bundle gets a real rule file + =CLAUDE.md=. Building out that bundle is the natural companion task. Also: modern =go test ./...= already lists every module package in the profile at 0%, so the missing-file list is usually empty for in-module code; it earns its keep on build-tagged files and dirs outside =./...=.
 
 ** TODO [#B] Cross-project pattern catalog :spec:thinking:
 :PROPERTIES: