fix(release): read checksum manifests the way coreutils writes them

[justinjoy]

Closes #1315. Stacked on #1314 (fix/1311-checksum-escaping) — review that first.

The bug

verify-release.sh rejected any archive whose name coreutils escapes, before comparing a
single hash:

checksum manifest does not name back\slash.tar.gz: back\\slash.tar.gz

GNU coreutils escapes exactly three characters in a name — \ → \\, newline → \n,
carriage return → \r — and prefixes the whole line with a literal \.

The escape set came from scanning all 254 legal filename bytes through sha256sum, not
from documentation. My first version had only two: I'd derived the set from the two cases
the issue happened to name, and missed CR. The reviewer caught it by running the scan I
should have run — an incomplete set treated as complete, which is the same defect as the bug
being fixed, reproduced inside the fix. Tab, other control bytes and UTF-8 are not escaped;
b3sum escapes the same three with the same marker.

A third failure had nothing to do with escaping. The name was read as awk '$2', and
coreutils does not escape spaces, so has space.tar.gz was truncated to has. The name is
now taken as everything after the two-character separator, which is also correct for binary
mode (hash *name).

Why escaping the expected name, not unescaping the manifest's

The issue suggested unescaping. That direction is ambiguous, and I only found out by
measuring it: a file genuinely named a\nb.tar.gz is written a\\nb.tar.gz, and replacing
\\ then \n in sequence corrupts it to a<newline>b.tar.gz. Escaping is one-way and
has no such case — which is why a\nb.tar.gz is one of the regression cases rather than an
incidental one.

Fail-closed, verified

The name check only selects which manifest line is trusted; the hash is recomputed
independently. Reviewer confirmed no input produces a false "verified" — corrupt archive with
an escaped name, a forged name field carrying another file's hash, multi-line manifests with a
wrong first line, empty and hash-only manifests are all rejected. It also checked that
escaping stays injective across all three rules, so a manifest for an LF-named file is
correctly rejected against a CR-named archive.

Tests

Five cases in scripts/ci/test-release-roundtrip.sh, built from real sha256sum/b3sum
output — a hand-written unescaped manifest would pin the parsing without exercising the
escaping condition at all.

variant	suite result
this PR	0 FAIL
minus the \r rule	1 FAIL — carriage return only
minus the \n rule	1 FAIL — newline only
minus the \\ rule	2 FAIL — backslash, literal \n
backslash rule moved last	2 FAIL — newline, carriage return
unfixed verify-release.sh	5 FAIL — all five

Each rule is independently pinned, and the ordering the comment asserts is executable
rather than prose. Verified on real bash 3.2.57 (the macOS floor) as well as 5.3, with no
temp-directory leftovers.

This also makes #1311's leading-backslash strip in hash_of reachable for the first time and
pins it, so the comment declaring it deliberately unpinned is replaced.

Left out deliberately

• make-tarball.sh's missing --/CDPATH= hardening → that is make-tarball.sh: manifest writing lacks the CDPATH/--/-P hardening applied to the reading side #1316, open and
  unassigned. It has no matching escaping defect: the archive name is machine-generated behind
  a version regex, so nothing escapable can reach its manifests.
• An archive name ending in a newline is still truncated, by basename inside a command
  substitution — a different line and a different fix. Filed as verify-release.sh: an archive name ending in a newline is truncated before the manifest comparison #1329 rather than folded
  into make-tarball.sh: manifest writing lacks the CDPATH/--/-P hardening applied to the reading side #1316, on the reviewer's argument that make-tarball.sh: manifest writing lacks the CDPATH/--/-P hardening applied to the reading side #1316 is scoped to the writing side and could be
  closed while leaving this unfixed and untracked.

Local: 306 Ok / 0 Fail / 12 Skipped, serialized, dedicated build dir.