Validate manifest entry paths on deserialize (closes #61)

Untrusted .mf files were parsed with no path validation, so an entry
like ../../etc/passwd flowed into filepath.Join against the checker's
base path. ValidatePath ran only on the write side (Builder). Enforce it
on every entry in deserializeInner, so all consumers — Checker now, any
restore/extract path later — are protected. Reject the whole manifest on
the first offender rather than dropping entries, which could hide files
from a check; failures wrap a new errInvalidManifestPath sentinel and
name the path.

A path that is not valid UTF-8 is refused at load time by the proto3
string decoder before validation runs; the test covers that plus the
traversal, absolute, backslash, and empty cases, and a valid round-trip.

Model: opus-4-8

TODO.md

@@ -24,6 +24,8 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 
 # Completed Steps
 
+- 2026-09-21: validate manifest entry paths on deserialize so untrusted `.mf`
+  files cannot make `Checker` stat or read outside `basePath` (#61)
 - 2026-08-09: added `.prettierrc`/`.prettierignore`, gave `script/fmt` and
   `script/fmt-check` one shared prettier file set via `script/prettier`, dropped
   the `|| true` that hid prettier failures, and added a node-based Dockerfile