excretion: feed digestibility is species-blind, inflating monogastric volatile solids

## What

`.feed_quality_digestibility()` in `R/excretion.R` assigns one digestibility
per feed quality for **every** species:

```
lactation 0.72, high_quality 0.72, low_quality 0.62, residues 0.47,
grass 0.62, substitute 0.62, zoot_fixed 0.68, scavenging 0.55
```

Its own comment marks it CALIBRATE and says it is "a per-feed-quality
scaffold anchored on GLEAM". Digestibility is species-specific as much as
feed-specific: a pig or a broiler on a cereal-soy diet digests 0.80-0.88 of
the dry matter, where 0.72 is a ruminant concentrate value.

## Why it matters

Volatile solids are `intake_dm * (1 - digestibility) * (1 - ash)`, so the
indigestible fraction is the whole quantity. Using 0.28 where 0.15 is right
roughly **doubles** monogastric VS, and since #1006 excreted carbon is
`vs_excretion * 0.47`, it doubles their excreted carbon too.

Measured on the 2020 national chain, implied whole-excreta C:N against ASABE
D384.2 as-excreted VS/N at the same 0.47 kg C/kg VS:

| | WHEP | ASABE D384.2 |
|---|---|---|
| Pigs | 10.8 | 4.7-6.6 |
| Poultry | 8.1 | 4.8-8.2 |
| Cattle (meat / milk) | 10.3 / 12.1 | 5.5-14.6 |

Cattle and poultry land inside the published range. Pigs are about twice the
top of theirs, and pig VS is the likeliest cause. WHEP pigs excrete 249 Tg VS
and 117 Tg C at 2020, so a factor of two here is ~50 Tg C on the manure
carbon reaching cropland soil.

## Suggested fix

Make digestibility depend on `(species_gen, feed_quality)` rather than
`feed_quality` alone, sourced per species (GLEAM for ruminants; a monogastric
source for pigs and poultry). Keep it selectable, and record the choice, as
the package requires for multi-method functions. Validate against ASABE
D384.2 VS/N by species, which is an independent check the package does not
currently make.

## Classification

**Science decision** — a coefficient table that moves published VS, manure
CH4 and manure carbon. Needs sourced values.

Found while fixing #1006. Refs #1006.

Start in R/excretion.R at .feed_quality_digestibility() and read its CALIBRATE comment and current feed-quality table. Find sourced digestibility values for each species and feed quality, keeping the choice selectable and recorded as required for multi-method functions. Validate the resulting VS/N by species against ASABE D384.2, including pigs, poultry, and cattle.

Written by the indexing model from the issue text.