Make tag-triggered publication restartable after a partial crates.io
release. The publisher skips exact crate versions already visible in the
sparse index, retries recognized transient upload failures only after
reconciling sparse-index visibility, and permits the exact tag version
during the workflow version test.

For example, if crates A 0.52.1 and B 0.52.1 publish, then uploading C
0.52.1 returns HTTP 503, a rerun skips A and B and continues with C. If
C became visible despite the 503, the publisher observes it and
continues without uploading C again.

Pushed release tags are immutable: a rerun may skip an already-visible
exact crate version based on sparse-index presence alone. Fix forward
with a new patch tag instead of moving an existing tag.

Tests: release-helper pytest suite; xlsynth-test-helpers version tests.

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- ➡ #993

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