test: coverage wave 2 — utility packages toward 95%

[LukasWodka]

Summary

Wave 2 of the coverage push (now on merged develop), driving the small self-contained packages with real, measured tests:

Package	Before	After
pathutil	91.7%	100%	✅
slug	94.3%	97.1%	✅
schema	84.5%	94.4%	plateau
config	75.8%	93.4%	plateau

pathutil + slug clear 95%. config + schema plateau just under — and it's worth being explicit about why, because it's the crux of "95% for all":

Both are blocked only by by-construction-uncoverable branches:

• config: the atomic-write's defensive I/O errors (Chmod/Write/Close on a just-created temp file) + a provably-unreachable MarshalIndent check (a *Config of strings/maps can't fail to marshal).
• schema: NewV1Validator's "embedded schema is malformed" defense-in-depth (can't happen — the CI drift check + go:embed guarantee a valid schema) + ValidateYAML's "jsonschema/v6 always returns a *ValidationError" defensive else.

Reaching those last statements would mean faking the filesystem / removing idiomatic error handling purely for the number — deliberately not done. Everything reachable in both packages is now covered.

Test plan

• make ci green. All additions are behavior-asserting (not number-padding); the one redundant test I initially wrote (it added 0 coverage) was removed.

🤖 Generated with Claude Code

---

Note

Low Risk
Test-only additions with no changes to production logic or runtime behavior.

Overview
Adds five*_coverage_test.go files—no production code changes—as wave 2 of pushing utility packages toward ~95% coverage.

internal/config exercises Current/Profile lazy map behavior, default vs missing-home Dir/Path, v2 load failures and nil profiles, v1 migrate parse errors, Save when home is unset or config.json is a non-empty directory, and clearAll error propagation (via TRACEBLOC_CONFIG_DIR / empty HOME).

internal/pathutil, internal/schema, and internal/slug add targeted cases for ExpandHome fallbacks, error-chain helpers and FormatErrors tie-break / nil flatten, and Derive empty-input, fallback, collision, and max-length truncation paths.

Tests assert observable behavior rather than padding metrics; remaining gaps in config/schema are documented as hard-to-hit defensive branches.

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[LukasWodka]

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[saadqbal]

LGTM 👍 Verified the specific ones against source — slug.Derive (raw-fallback when both slugify empty, the -2 collision suffix, and the max-length truncation keeping len ≤ MaxLabelLength) and config.Current/Profile/Dir error arms all line up. Clean branch-coverage additions across the utility pkgs.