fix #12: silent-empty output when --root is relative

[oubiwann]

Closes #12.

What

cascade v0.1.0's documented invocation cascade --base=origin/main --head=HEAD (no --root, taking default .) silently emitted empty output and exited 0 against any real Go monorepo. The exact silent-zero-packages failure mode that drove the gta → cascade pivot.

Three-layer holistic fix (per plan-mode user decisions for defense-in-depth + diagnostic loudness):

1. Library — pkg/changeset/changeset.go: absolutize cfg.moduleRoot via filepath.Abs before the dirMap lookup. Defensive at the library level — protects ANY current/future caller (deferred pkg/cascade.Run; third-party imports of pkg/changeset).
2. CLI — internal/cli/cli.go: absolutize cfg.root once after parseFlags succeeds. Surgical fix; the four downstream consumers all receive a path absolutized exactly once.
3. Diagnostic — internal/cli/cli.go: stderr breadcrumb when len(affected) == 0 and changedFiles contains .go-suffix entries. Filtered on .go so docs-only PRs (legitimate empty case) stay silent. Doesn't change exit code.

Where

Path	Action	Notes
pkg/changeset/changeset.go	Modify	filepath.Abs(cfg.moduleRoot) after the moduleRootSet check; updated WithModuleRoot godoc to document the new behaviour.
pkg/changeset/changeset_test.go	Modify	New TestResolve_RelativeModuleRoot_AbsolutizedInternally (subtests for . and "").
internal/cli/cli.go	Modify	filepath.Abs(cfg.root) after parseFlags; diagnostic-warning block in Run after runPipeline.
internal/cli/seam_test.go	Modify	3 new tests: TestRun_DefaultRootResolvesCWD, TestRun_DiagnosticOnSuspiciousEmpty, TestRun_NoDiagnosticOnDocsOnlyEmpty.
README.md	Modify	--root flag row clarifies absolutization.
docs/dev/0013-bug12-fix-relative-root.md	Create	Closing retrospective.

Root cause (verified)

Location	What goes wrong
internal/cli/cli.go:189	Flag default is the literal string ".".
internal/cli/cli.go:279	changeset.WithModuleRoot(cfg.root) hands the relative "." to the library.
pkg/changeset/changeset.go:120	dirMap[p.Dir] = p.ImportPath — keyed on absolute paths (per golist.Package.Dir's contract).
pkg/changeset/changeset.go:136	filepath.Join(".", "rel/path") returns relative — mismatch.
pkg/changeset/changeset.go:139	Lookup miss → no seeds → empty RevDepClosure([]) → empty stdout, exit 0.

The bug lived in a parameter-space cell that no test happened to hit (relative moduleRoot × absolute pkg.Dir). Both M4 and M5 had 100% statement coverage but the pairing was untested in either layer.

How to verify

# Reproduce the bug locally (against this branch — should pass post-fix):
make check-all                                   # 100% on all 5 packages, no contortions
go test -run TestResolve_RelativeModuleRoot ./pkg/changeset       # library-layer regression
go test -run TestRun_DefaultRootResolvesCWD ./internal/cli        # CLI-layer regression
go test -run 'TestRun_(Diagnostic|NoDiagnostic)' ./internal/cli   # diagnostic behaviour

# Smoke test against cascade-against-cascade with default --root
# (would have produced empty output on v0.1.0; produces 5-package affected-set on this branch):
go build -o /tmp/cascade-fixed ./cmd/cascade
echo "pkg/golist/golist.go" | /tmp/cascade-fixed --changed-files=-
# Expected: github.com/geomyidia/cascade/{cmd/cascade,internal/cli,pkg/{changeset,depgraph,golist}}

Tests added

Test	Layer	What it pins
TestResolve_RelativeModuleRoot_AbsolutizedInternally	pkg/changeset	Library-layer regression. Subtests for . and "". Pre-fix fails (empty); post-fix passes.
TestRun_DefaultRootResolvesCWD	internal/cli	End-to-end CLI integration — cli.Run with no --root flag against absolute pkg.Dir values. Pre-fix produced bug; post-fix produces affected-set.
TestRun_DiagnosticOnSuspiciousEmpty	internal/cli	Diagnostic stderr line fires when .go files in changedFiles + empty affected-set. Pins the count format too.
TestRun_NoDiagnosticOnDocsOnlyEmpty	internal/cli	Diagnostic does NOT fire on docs-only changesets. Pins the .go-suffix filter.

Notable findings

• Behavioral-space gap as methodology lesson. M5 retro claimed "100% coverage on first try, no test contortions" — accurate for line coverage, but coverage didn't catch this bug. The bug lived in a parameter-space cell that no test happened to hit. Documented in the retrospective as a carry-forward: future tests for path-handling code should explicitly enumerate the relative-vs-absolute matrix.
• CC self-check protocol gap exposed. The 5-for-5 zero-softpedals streak across M3/refactor/M4/M5/M6 is for textual softpedals (evidence-vs-criterion mismatches). Doesn't extend to behavioral-space gaps. Worth recording as a known limitation.
• Five-seam pattern paid off for regression coverage. All 4 new tests drive the scenarios in-process via existing seams (runGitDiff, runGoListWrapper, signalContext); no real subprocess invocations.

Post-merge follow-ups

• Tag v0.1.1 (manual tag + push to bypass the Makefile's release-dry-run go.sum bug).
• gh release create v0.1.1 with notes calling out the fix + reproduction case + verification PR.
• Re-trigger getBanyan/api PR #3474 against v0.1.1 — load-bearing real-codebase closure for bug v0.1.0: silent empty output when --root is relative (default '.' is broken in real-world CI use) #12 (M2 F-18 / M5 F-19 lineage).
• Append v0.1.1 install + reproduction evidence to the retrospective.

Checklist

• make check passes locally
• All 5 cascade packages at 100% coverage post-fix
• New/changed exported symbols have godoc comments (only WithModuleRoot's doc-comment changed; updated to document the absolutize behaviour)
• Tests added/updated for behaviour change (4 new tests across two packages)
• No public-API breakage (the WithModuleRoot documented contract is refined, not changed: relative paths now resolve correctly instead of silently failing)
• If this PR closes a milestone ledger — N/A (bug fix, not a milestone). Both pre-PR self-check + the retrospective walk the three-layer fix's evidence-vs-claim matching.

Breaking change?

No. The library-layer change refines WithModuleRoot's documented behaviour (relative paths now resolve correctly via filepath.Abs) without changing any contractual guarantee. Tests passing absolute paths see no observable change (absolutize-of-absolute is identity). The CLI-layer change makes --root=. (the default) work as documented; users who had been working around the bug by passing absolute --root see no observable change either.

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