Add a seeded robustness harness for the untrusted-input parsers

Five parsers that take bytes straight off a disc are now swept with
generated input asserting one property: they return Ok or Err and never
panic. That is this crate's own hard rule, and the class seven rounds of
reading is worst at.

Written in-crate rather than with cargo-fuzz, which needs a nightly
toolchain this project does not use, and without proptest or arbitrary,
because one dev-dependency is a deliberate posture and the parsers take
plain byte slices. What is given up is coverage-guided mutation, which is
the real loss. What is gained is determinism: the same seed replays the
same cases anywhere, so a CI failure reproduces locally verbatim.

Three generators, and the second is the one that matters. Pure random
bytes die at the magic check and exercise the entry guards only; prefixing
valid magic is what reaches the parser body; mutating a mostly-zero record
is what reaches the offset and count arithmetic a hostile image would lie
about.

That claim is MEASURED, not asserted. A harness whose cases all bounce off
the entry guards is the fuzzing equivalent of a test that cannot fail, so
one test counts how many generated cases parse to completion: 15,606 of
60,000, about 26%. If a future change to a guard drops that to zero, the
test fails rather than continuing to report a meaningless pass. Two further
tests pin that the three generators produce different bytes and that a seed
replays identically.

1.2M cases across all five targets found nothing. On this evidence that is
a real negative rather than an empty one.

The first version of this file was itself broken in the way this audit
keeps finding: its two meta-tests set FREEMKV_HARNESS_CASES and raced,
because the test harness runs them in parallel and env mutation is unsound
there. The budget is a parameter now, and the environment is read once at
the call site.

Two crate-internal parsers widened from private to pub(crate) so the
harness can reach them. No public API change.

.gitignore

@@ -14,3 +14,8 @@ scratch/
 # internal agent context — never publish (path AND dir; leak-guard blocks both)
 CLAUDE.md
 .claude/
+
+# Nightly harness output. Written into the repo it audits, and it embeds
+# absolute paths from the machine that ran it — which must never reach a public
+# repo. Ignored rather than relocated so a run from any working copy is safe.
+.nightly/