serve a declared slug as an address, leaving the key alone

The human chose the second option: a route sits beside the key rather than
replacing it. So `slug` renames what a bundle is served at, in every language, and
identity stays derived from the path — which is exactly what keeps ADR-0033 intact,
since series membership is the directory. A series landing page can now be renamed
without orphaning its chapters, and there is a test that says so.

`Site` resolves routes at index time, because only it can see whether every variant
agrees. Disagreement is dropped rather than resolved, as is a slug landing where
another bundle already answers — the same rule colliding keys and contested aliases
already follow. The key a slug moved away from stops answering, so the old address
does not quietly keep working.

Two bugs surfaced doing this, both older than this change:

An alias naming its own bundle's former key was rejected as "an alias that names a
real bundle" — which made rename-plus-alias, the entire point of ADR-0008's alias
mechanism, impossible. The check now asks what a request asks: is anything actually
served there.

Aliases were counted per declaring *file*, so a bundle whose two language variants
both listed the same alias looked like two rival claimants and lost the alias. It is
a set of keys now. This one only appears with translated content, which is why no
fixture had caught it since entry 4 — the real binary did, on the first multilingual
rename.

docs/theme-contract.md

@@ -15,7 +15,7 @@ A bundle page receives:
 |---|---|
 | `.Title` | may be empty; a template falls back to `.Key` rather than failing |
 | `.Lang` | the locale of this variant, always set |
-| `.Key` | the bundle's identity, without language or extension |
+| `.Key` | the bundle's identity, without language or extension. **Not its address** — a `slug` moves the address and leaves the key alone, so link with `.URL` or `.Canonical` and never by assembling a key (ADR-0035) |
 | `.HTML` | the rendered body, already escaped |
 | `.Extra` | every frontmatter key the parser does not name (ADR-0002) |
 | `.Style` | the reference theme's stylesheet, inlined so a bare site root needs no asset route |