# Fix hour() setter clamping to an invalid hour (24)

## Problem

Calling `.hour(n)` with `n >= 24` produces an invalid spacetime whose epoch is
`null`, instead of clamping to a valid hour:

```js
const s = spacetime('2021-03-14 10:00:00', 'UTC')

s.hour(24).isValid()      // false
s.hour(24).epoch          // null
s.hour(25).format('iso')  // '' (empty - object is invalid)
```

This is inconsistent with every other overflow-clamping setter:

- `.date(40)` clamps to the last day of the month,
- `.month(13)` clamps to December (11),
- `.hour(-1)` clamps to 0.

Only the upper bound of `.hour()` is broken.

## Root cause

`src/methods/set/set.js`, in `hours()`:

```js
if (n >= 24) {
  n = 24
} else if (n < 0) {
  n = 0
}
```

Valid hours are `0-23`. Clamping to `24` yields a value that
`src/methods/set/walk.js` rejects (`hour.valid: (n) => n >= 0 && n < 24`), and
on an invalid unit `walkTo()` sets `s.epoch = null`, invalidating the whole
object. The negative branch is already correct (clamps to the valid `0`); the
`>= 24` branch is off by one and should clamp to `23`.

## Fix

```js
if (n >= 24) {
  n = 23
} else if (n < 0) {
  n = 0
}
```

`hour(24)`, `hour(25)`, `hour(100)` now all clamp to `23:00` on the same day,
matching the behavior of the other setters.

## Test

Added a test to `test/set.test.js` (next to the existing month/date clamp test)
asserting that out-of-range hours clamp to `23`, keep the object valid, keep the
same day, and that negatives still clamp to `0`.

Counterfactual (revert `n = 23` back to `n = 24`):

```
not ok - hour(24) clamps to 23
  operator: equal
  expected: 23
  actual:   0
```

(the invalid object reports hour `0`).

## Gate

- `npm test` (tape) run against `src` (the default dev target of
  `test/lib/index.js`): the new test passes; the full suite shows only 18
  pre-existing DST/timezone failures (`europe/london`, `atlantic/madeira`,
  `atlantic/azores`, some date-dependent "dst-now") that are present verbatim
  without this change and are unrelated to it.
- `npm run lint` (eslint ./src): exit 0.