fix(release): read the crate checksum from the sparse index

[us]

publish_crate.sh treats "already uploaded" as success only when the local
.crate sha matches the published one. That check has been silently broken:
crates.io's /api/v1/crates/<name>/<version> response no longer carries
version.cksum.

Checked against the live registry on 2026-09-10, the field is absent for every
crate:

$ curl -fsSL https://crates.io/api/v1/crates/crw-mcp-proto/0.34.0 | jq '.version.cksum'
null

So crate_version_cksum returned an empty string with rc 0, and the caller
compared it straight against the local sha. Every idempotent re-publish therefore
failed as a content mismatch. The v0.34.0 release (run 34490023731) died there:

error: crate crw-mcp-proto@0.34.0 already exists on crates.io index
crw-mcp-proto@0.34.0 content mismatch - local=8e9006f12b5e8da3434b167991cc75b6168e1c32f07eb17ee5b54201e00d3b3c remote=
crates.io is immutable; cannot republish. Bump the version.

The sparse index reports that same 8e9006f12b... for the version, so the upload
matched and the run should have taken the idempotent-skip path. The advice to
bump the version was wrong, and would have burned a version number over a lookup
that simply failed.

Change

• crate_version_cksum reads the checksum from index.crates.io (the registry
  index protocol, where the value is authoritative) instead of the v1 API, and
  returns non-zero when the index or the version cannot be read rather than
  printing nothing and succeeding. crate_index_path implements the documented
  1/2/3/4+ prefix layout.
• publish_crate.sh separates "unknown" from "different". An unverifiable upload
  still fails the release, because blessing it would defeat the guard, but it no
  longer reports a content mismatch or advises a version bump when the checksum
  could not be read.
• crate_version_present is unchanged: it reads .version.num, which the v1 API
  still returns.

Tests

Six tests added to the existing guard suite (scripts/release/test_guards.py,
already run by CI). They stub curl and cargo on PATH so the real scripts run
with no network:

test_crate_index_path_follows_the_registry_protocol
test_crate_cksum_reads_the_requested_version_from_the_index
test_crate_cksum_fails_when_the_version_is_absent
test_crate_cksum_fails_when_the_index_is_unreachable
test_unreadable_cksum_is_not_reported_as_a_content_mismatch
test_matching_cksum_is_an_idempotent_skip

All six fail against the code on main and pass with this change; the suite is
14/14 locally.

Deploy notes

Release-pipeline only, no engine or API surface is touched. The next release that
hits the already-uploaded path takes the idempotent-skip branch instead of
failing, which is what publish-crates needed on the last two runs.