fix: allow safe relative subfolders in filename while still blocking traversal

[artemo-brd]

Summary

Follow-up to the path-traversal fix (CWE-22) already on main. That fix reduced anyfilename to its bare basename via path.basename() — safe, but more aggressive than necessary: it also stripped legitimate relative subfolders developers commonly use (e.g. filename: 'output/data.json'), even though those never posed a traversal risk. Since that basename-only behavior was never published to npm, this PR is what users will actually see as the shipped behavior.

What changed

FilenameSchema (src/schemas/shared.ts) — now allows relative subfolder segments (output/data.json, a/b/c/file.txt), and instead explicitly rejects, with a clear ValidationError (not a silent rewrite):

• ".." segments anywhere in the path
• absolute paths — POSIX (/etc/x), Windows (C:\x), UNC (\\host\share)

Backslashes are always treated as separators (not just on Windows), so a Windows-shaped traversal payload can't slip through on a POSIX host by switching separator style.

getAbsAndEnsureDir (src/utils/files.ts) — the single chokepoint all three write paths converge on (saveResults, SnapshotAPI.download, and BaseResult.save() — the last of which accepts a raw path that never passes through FilenameSchema at all) — now enforces containment itself rather than assuming callers already sanitized:

1. Lexical containment: the resolved candidate must stay under the base directory (cwd by default).
2. Real-path containment: after mkdir, re-resolve via fs.realpath and re-check, so a subfolder that is (or contains) a symlink pointing outside the base directory can't be used to escape it — lexical resolution alone is blind to symlinks.

Containment is only enforced for relative inputs. An absolute path is a deliberate choice by the calling code (same trust model as calling fs.writeFile directly) — BaseResult.save() has always supported writing to an arbitrary absolute destination (see the pre-existing /tmp/... test in tests/result.test.ts), and that's preserved unchanged.

User-facing behavior (vs. the currently-published 1.1.0)

Input to filename/filepath	saveResults / SnapshotAPI.download	result.save()
'output/data.json' (relative subfolder)	✅ creates output/ and writes there	✅ same as always
'../etc/passwd', 'output/../../x'	❌ ValidationError (clear, fails fast)	❌ FSError (containment check)
'/etc/passwd', 'C:\x', '\\\\host\\share'	❌ ValidationError	✅ honored (explicit, trusted absolute path — unchanged)
Subfolder that's a symlink escaping cwd	n/a (blocked earlier by schema for .., but a non-..-shaped symlinked folder is still caught)	❌ FSError (symlink escape detected via fs.realpath)

Tests

Rewrote tests/files.test.ts:

• FilenameSchema suite for the new allow-subfolders/reject-traversal behavior.
• New dedicated getAbsAndEnsureDir suite: nested subfolders, relative-traversal rejection, explicit-absolute-path pass-through, symlink-escape rejection.
• Extended saveResults integration tests: nested subfolder write, symlink-escape rejection.
• New BaseResult.save() coverage for the schema-bypass path (still enforces relative-traversal/symlink protection, still honors absolute paths).
• Symlink-creating tests are skipped on Windows (fs.symlink() needs Developer Mode/admin there — an environment limitation, not a regression) via it.skipIf.

Verification

• sanity (lint + typecheck): clean.
• Full suite: 396/408 passing (12 skipped — real-API integration tests).
• build + smoke-dist (ESM/CJS load from actual dist/): clean.