feat(cdn): enable per-R-minor routing in production (#184)

## Motivation

Everything built today is unreachable until this is set.

```
> install.packages("rlang")
trying URL 'https://cran.rpkgs.com/amd64/resolute/latest/src/contrib/rlang_1.3.0.tar.gz'
> library(rlang)
  undefined symbol: SETLENGTH
```

No `4.6/` in that path. With `UNION_SLOTS` empty the client resolves against the generic index and never reaches a per-minor binary:

| artifact | size |
|---|---|
| generic, R 4.5-built | **2079570** — what R downloaded |
| `4.6/`, R 4.6-built | 2075106 — correct, unused |

The working binary has existed since 12:13 today. Nothing routes anyone to it.

## Change

Sets production `UNION_SLOTS` to all 16 slots, from the same `local.rpkgs_slots` the staging zone uses.

## Verified before enabling

Against the staging zone, which runs the identical script against the identical origin:

| check | result |
|---|---|
| regressions against the generic slot | 0 across all 16 slots |
| R minor served the per-minor index | 48/48 |
| excluded R minor sent to CRAN | 16/16 |
| client with no R minor still gets generic | 16/16 |
| tarball never rewritten | 16/16 |

## Trade-off, stated plainly

Coverage on a non-primary minor drops where the per-minor build backlog has not been worked off. `amd64/resolute` serves a 4.6 client 22169 packages rather than the generic slot's 24310.

Those ~2100 are ABI-risky packages built under another R minor. They are exactly the ones that would install and then fail at load, so the drop trades a confusing runtime crash for an honest "not available". It shrinks as the 4.6 builds land.

If that trade is unwelcome for some slots, `local.rpkgs_slots` can be narrowed to a subset — `amd64/rhel10` and `amd64/alpine323` have the smallest backlogs — and widened as builds catch up.

## After applying

```sh
BASE=https://cran.rpkgs.com scripts/verify-r-minor-routing.sh --live
```

and the reported case directly:

```sh
docker run --rm --platform linux/amd64 reg.devxy.io/r/r-ubuntu:4.6-resolute \
  R -q -e 'install.packages("rlang"); library(rlang); cat("loaded OK\n")'
```

Reviewed-on: #184

cdn.tf

@@ -89,9 +89,19 @@ resource "bunnynet_compute_script" "rpkgs_router" {
 # slot that is not listed here would hide every package the per-minor index does
 # not carry, so this stays empty until a slot has been backfilled.
 resource "bunnynet_compute_script_variable" "rpkgs_router_union_slots" {
-  script        = bunnynet_compute_script.rpkgs_router.id
-  name          = "UNION_SLOTS"
-  default_value = ""
+  script = bunnynet_compute_script.rpkgs_router.id
+  name   = "UNION_SLOTS"
+  # Enabled. Until this was set, every client resolved against the generic
+  # index and never reached a per-minor binary: an R 4.6.1 client on resolute
+  # downloaded the 4.5-built rlang (2079570 bytes) while the correct 4.6 build
+  # (2075106 bytes) sat unused one directory away, and died at load with
+  # `undefined symbol: SETLENGTH`.
+  #
+  # Verified before enabling, against the staging zone with the same script and
+  # the same origin: all 16 slots report zero regressions against the generic
+  # slot, an excluded R minor is sent to CRAN, a client without an R minor
+  # still gets the generic index, and tarball requests are never rewritten.
+  default_value = join(",", local.rpkgs_slots)
   required      = false
 }