A MISSING_FROM_INDEX row has no install to time, and `%.1f` renders the NA as
the literal "NAs", which reads as a unit. Report the row without a timing.
## Why
Everything built so far reasons about the *binaries*: `abi_flat_safety` records what an object's symbols say it can load under, and the index writer drops records on that verdict.
None of it proves what a **client** actually receives, which is the product of the CDN rewrite, the per-minor indexes and the `Path:` field acting together.
The reprex that found the `base64enc` failure ran on a workstation, could only reach amd64, and did not survive the session.
This makes it a pipeline.
## What it does
For every slot, under every R minor the image carries, it installs a fixed package set through each URL form the website documents and checks that each one loads in a fresh subprocess.
| form | repository | user agent |
| --- | --- | --- |
| `arch` | `https://cran.rpkgs.com/<arch>/<codename>/latest` | R's own |
| `generic` | `https://cran.rpkgs.com/` | the string the website's snippet builds |
| `flat` | the arch URL | `curl/8.5.0`, so the edge cannot read an R minor |
`arch` and `generic` are contractual, so a failure there fails the run.
`flat` measures what a client hiding its R minor is exposed to: it is reported and never fatal, because pruning the flat slot is a separate decision.
Two details that matter:
- **CRAN is deliberately absent from `repos`.** A package missing from our index must fail loudly rather than be quietly compiled from source, which would hide exactly the gap this exists to find.
- **One process per form.** `available.packages()` caches its index in `tempdir()` keyed by repository URL, and `arch` and `flat` share a URL. In a single process the second form would read the first form's index and prove nothing.
## Verification
Run against the live repository in `build-env-ubuntu:noble` under R 4.6.0, mid-repair:
```
amd64/noble R 4.6 via arch: 11 of 11 packages usable
amd64/noble R 4.6 via generic: 11 of 11 packages usable
amd64/noble R 4.6 via flat: 8 of 11 packages usable
```
The three `flat` failures are `Rcpp`, `data.table` and `yaml`, each `Built: R 4.4.3` and each failing `dyn.load()` under 4.6.
That is the unrouted-client exposure, measured rather than estimated.
The rows also show the routed forms serving a mix of per-minor and flat objects (`Rcpp` at `R 4.6.0`, `stringi` at `R 4.4.3`), which is the intended behaviour: only objects that cannot load cross-minor get a per-minor copy.
Reviewed-on: #209
