Address the review on #14

[Maximilian-Nesslauer]

The review findings on #14, implemented on top of your branch. Five commits, one theme each, each green on its own.

Commit	What
Separators	Prose only, no behavior.
Stamper	The RFC 0035 release-time rules, plus id, spec_version and range validation.
Hosts	Every failure back inside the HostError and StampError split, and a URL that cannot leave its host.
Watcher	The tick's stated properties held under failure: the write guard, one listing never failing the tick, the ETag invariant, one issue per listing.
Workflow	The cache is saved only when the push landed.

54 tests to 106. Every fix has a regression test that fails without it, and the error paths get their first tests at all, which is why most of these got through: the existing tests assert the happy path of each RFC sentence.

Two things worth your call rather than mine:

• The month re-resolution is in, against what you wrote in Add the watcher and the stamper #14. It reads the authored document out of the checkout the tick already has, and only ever adds a bound, so it stays inside RFC 0031's never-more-permissive rule. Drop the commit's month_pass if you disagree.
• The first commit touches your comments. Separate on purpose so you can take the rest without it.

Take it, cherry-pick it, or treat it as a diff to read: whatever is least work for you.