fix: address changes run the loop simulation and validate their addresses (WR-09)

AddressChanged wrote its 301 and re-pointed rules without Problems or the
save-time simulation, and removed only an exact rule whose source equals
the new address literally. It now removes every exact rule that matches
the new address with its options, simulates the 301 and the re-pointing
with the stored rules and writes nothing when that loops, stores an
absolute old address on the site as its path, and skips an address that
is not a path on the site.

README.md

@@ -24,7 +24,7 @@ The frontend (a Nuxt layer or app) asks `GET /_seo/api/v1/resolve?url=...` for t
 - **Resolver.** Canonical URLs are self-referencing and absolute from the configured site URL, keep content parameters and drop filter and tracking parameters; overrides may be relative or absolute. Robots resolve per directive and are also answered as the `X-Robots-Tag` value. Open Graph and Twitter values inherit from the effective title, description and image. Verification meta tags come with the home page.
 - **Redirects** (`golem15_seo_redirects`): 301, 302 and 410 rules with exact, pattern (`/old/*` → `/new/*`) and regex (`^/p/(\d+)$` → `/post/$1`) sources, evaluated exact, then patterns (longest literal prefix first), then regex. Options: ignore letter case, match with and without a trailing slash, ignore or pass the query string; a source with `?` matches its query exactly and wins over the same path without a query. Regex sources use the standard library (RE2, linear time) and are capped at 512 characters. A capture never turns a path target into another host, and an absolute target keeps its own host.
 - **Loops and chains.** Every redirect write runs a simulation with the rule in place (a probe URL for patterns and regex, at most 10 hops): a loop is refused, from the admin with `redirects.loop` on the target and nothing saved; a chain is saved with `redirects.chain_notice` naming the final target and is marked in the list.
-- **Address changes.** When a published record's address changes, a 301 from the old to the new address is created, earlier redirects to the old address are pointed straight at the new one, and a redirect whose source is the new address is removed (a slug changed back and forth ends with one redirect per old address). In the admin this happens inside the save transaction and the editor gets `redirects.auto_created` on the slug field; writes outside the admin (Journal's API, MCP, imports) are handled after commit through `pact.RecordSaved`. Deleting or unpublishing a record creates nothing.
+- **Address changes.** When a published record's address changes, a 301 from the old to the new address is created, earlier redirects to the old address are pointed straight at the new one, and an exact redirect that matches the new address (with its case and trailing-slash options) is removed (a slug changed back and forth ends with one redirect per old address). The change is simulated first: when it would loop through a pattern or regex rule, or an address is not a path on the site, no redirect is written and a warning is logged. In the admin this happens inside the save transaction and the editor gets `redirects.auto_created` on the slug field; writes outside the admin (Journal's API, MCP, imports) are handled after commit through `pact.RecordSaved`. Deleting or unpublishing a record creates nothing.
 - **404 log** (`golem15_seo_notfound`): the frontend reports each 404 to `POST /_seo/api/v1/notfound`; the log keeps the address, hits, first seen, last hit and last referer. An entry is resolved with one click: **Create redirect…** (a prompted 301), **Mark as gone** (410), or a bulk 410.
 - **URL normalisation** (settings tab Addresses): https, `www` or bare host, trailing slash added or removed, optional lowercasing of ASCII letters, double slashes and `/index.html` collapsed. The defaults change nothing but scheme, slashes and index files.
 - **Strict URLs.** A path that names a record without being its address (a wrong category, a locale prefix with another locale's slug) 301s to the record's address; a wrong URL never answers `ok`.