fix(set): clamp hour() to 23 instead of the invalid 24

hour(n) with n >= 24 clamped n to 24, which walkTo() rejects as an invalid
hour (valid range 0-23), nulling the epoch and invalidating the object.
Clamp to 23 to match the other overflow-clamping setters (date, month).

PR.md

@@ -0,0 +1,79 @@
+# 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.