Add revision-aware CMS taxonomies and relationships

docs/CMS.md

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+<!-- SPDX-License-Identifier: MPL-2.0 -->
+
+# Classification without a page builder
+
+`cms` defines small editorial values. `cmssqlite` stores them in caller-owned
+SQLite transactions. Neither package owns your content, router, templates,
+permissions or billing model. Use them alongside typed Go records and coded
+templates, not as a database-defined application builder.
+
+## Two distinct relationships
+
+- A **taxonomy** names a flat vocabulary, such as Categories or Topics. A **term**
+  has a stable ID, editable name/description and an address. Renaming its address
+  preserves aliases; retiring it hides discovery without rewriting old revisions.
+- An **explicit link** joins two `{kind, id}` references. `Related` reads that
+  single edge in either direction. Sharing a term alone does not assert a link.
+
+References contain no titles, URLs, application data or authorization. Resolve
+each public result through the owning repository using its current published
+revision and availability. Never render the latest draft merely because an older
+revision is published. Products may have additional availability/approval rules;
+editorial links do not bypass them or change prices, entitlements or purchases.
+
+## Transactions and publication
+
+Call `CreateSchema(ctx, tx)` during an explicit application migration. It creates
+the `gwf_cms_*` tables, independently of the authentication adapter's schema.
+There is no automatic migration, connection, worker or request middleware.
+Enable foreign keys on every connection and use the application's existing
+SQLite write discipline. Keep backups and schema compatibility in that owner.
+
+Every reader/writer takes a validated namespace. A namespace separates data; it
+does not authorize the caller. Check permissions before reads and inside the
+application's mutation boundary where concurrent revocation matters.
+
+Within the same transaction as your content revision and audit:
+
+1. `PutRevision` stores the exact revision's immutable term/link selections.
+2. For publication, `SetPublished` points at that revision. Zero unpublishes.
+3. Commit content, associations, publication and audit together.
+
+Saving a draft does not advance publication. Restore by copying the selected
+historical association document into a new revision, then publish separately.
+Terms must exist in the namespace; retired terms remain valid historical values.
+Applications decide which retired selections may be retained in new edits.
+External targets can be indexed with an empty published snapshot, but their
+owning module still determines whether a public link is available.
+
+Taxonomy/term writes use expected revisions (zero for creation). Keep immutable
+IDs across name changes. Taxonomy addresses are fixed after creation; term
+addresses retain redirect history. A collision or stale revision returns
+`cms.ErrConflict`, not a successful overwrite. Transactions must be rolled back
+after any mutation error, including a later application audit failure.
+
+## Bounds and discovery
+
+- Each snapshot accepts at most 24 distinct terms and 16 distinct links, without
+  self-links. Validation rejects invalid IDs, control characters and duplicate
+  selections. Text fields have explicit byte bounds; names are not HTML.
+- A namespace has at most 100 taxonomies. `Terms` pages by stable term ID, at
+  most 200 results per request. Applications should choose their own overall
+  editor limits and search UI rather than loading an unbounded catalog.
+- `Members` and `Related` return at most 100 published references per page,
+  ordered by kind/ID. Pass `Next` for the following page. Publication filtering
+  happens before pagination; never use a mutable title as a cursor.
+- Owning-module visibility can filter further. Continue fetching bounded index
+  pages to fill a visible page and construct a cursor from the last visible
+  item; do not expose private titles or identifiers through error messages.
+
+The package tests use real SQLite transactions, including WAL, race execution,
+scope isolation, delayed publication, reverse discovery, aliases, retirement,
+pagination and rollback. Consumer tests still need to prove actual HTTP/API
+permissions, public visibility, escaping, editor usability and application data
+preservation. These packages do not claim to provide an entire CMS.