fix: pds check <file> resolves the workspace root (refs #67)

[starscream-bot]

Problem

pds check main.pds exits 1 with walking ``` IO error; only pds check

` worked (refs #67).

Cause

find_root walked the ancestors of a relative path. The chain bottoms out at the empty path ""; "".join("pds.toml") resolves against the cwd, so the probe matched and "" was returned as the root — then handed to WalkDir, which fails on an empty path. Affected any relative .pds argument; a relative root above the cwd was also unreachable.

Fix

Absolutise start (lexically, no disk access) before the ancestor walk. The walk now climbs the real tree to the filesystem root; no empty-path ancestor.

Verified

• pds check main.pds, pds check pkg/svc.pds, pds check . — all exit 0 from inside a workspace.
• Unit: find_root over a root-level file, a nested file, and a no-manifest tree.
• Feature: pds check <bare-name> from the workspace cwd (the exact repro).
• cargo test (pseudoscript, pseudoscript-project), clippy, rustfmt, model_conformance — green.

🤖 Generated with Claude Code

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[shockwave-bot]

Shockwave — verification pass

Scope: 3 files, +71/−5. Fix measured correct.

Confirmed

• std::path::absolute lexical, rooted at / — no empty-path ancestor. Edition 2024 (≥1.85) ⇒ absolute (stable 1.79) available. No MSRV break.
• FQN derivation unaffected — load_modules strips root prefix lexically (fs.rs:62); absolute root does not perturb module names.
• 9 find_root callers (main.rs ×5, deps.rs ×4) consume root via .join(...) — absolute root valid for every site. None assumed a relative root.
• Tests sound — tempdir inputs already absolute, absolute() lexical ⇒ no symlink//private/var flake.

Finding (low, non-blocking)

• crates/pseudoscript/src/main.rs:393 (check_one_workspace) — diagnostic prefix {root.display()}: … changes relative→absolute. pds check . / pds check <reldir> previously printed ./relative dir; now absolute. Not test-covered (check.feature:13 asserts contains <message>, not prefix), unmentioned in PR. Cosmetic CLI output change, no functional defect.

Verdict: PASS on correctness. Regression covered by 3 unit tests + bare-name repro scenario. One untested output-format change — flagged for awareness, non-blocking.