fix(timezone): keep a target wall clock the host DST gap cannot hold

[andr2k]

The bug

dayjs(instant).tz(zone) is an hour off whenever the target zone's wall clock
falls inside the host machine's spring-forward gap, even though the target
zone has no transition there.

// host TZ=America/Vancouver (or any US host: the gap is 2026-03-08 02:00-03:00)
const instant = new Date('2026-03-07T12:27:20Z')
dayjs(instant).tz('Pacific/Chatham').format('YYYY-MM-DD HH:mm')
// was:      2026-03-08 03:12   (and valueOf() an hour past the instant given)
// now:      2026-03-08 02:12   (matches moment-timezone and Intl.DateTimeFormat)

Two causes, both fixed here

1. proto.tz parsed a locale string in the host timezone.

const target = date.toLocaleString('en-US', { timeZone: timezone })
ins = d(target, { locale: this.$L }).$set(MS, this.$ms).utcOffset(offset, true)

d(target) hands those fields to new Date(string), which reads them as host
local time. When they name a nonexistent host-local time the engine shifts them
an hour forward, and utcOffset(offset, true) keeps the shifted clock.
tzOffset() already reads the same fields via Intl.DateTimeFormat.formatToParts,
so they are taken from there and the instance built with Date.UTC(...). The
toLocaleString round trip is gone, which also removes .tz()'s dependence on
how the running ICU formats and how the engine parses it.

2. Every instance with a display offset stored its wall clock in a host-local
Date, which cannot hold one the host itself skips.

This is the deeper one, and it is not in the timezone plugin at all:

// no timezone plugin loaded, host TZ=America/Vancouver
dayjs(new Date('2026-03-07T12:27:20Z')).utcOffset(825).format()
// was: 2026-03-08T03:12:20+13:45

With $u === false, init() reads the fields off $d with the local getters,
so $d had to be a host-local Date whose local fields are the wall clock -
and in a gap no such Date exists. Both candidate offsets and the
$x.$localOffset compensation were checked; neither has a solution. So offset
instances now keep their wall clock in the UTC fields of $d, which can
represent any wall clock a zone shows:

	before	after
$d	host-local, wall clock in local fields	wall clock in UTC fields
$u	false	true ($offset is what marks it non-UTC)
valueOf()	$d - ($offset + ($x.$localOffset || $d.getTimezoneOffset()))	$d - $offset
utcOffset()	$u checked before $offset	$offset checked before $u
isUTC()	!!$u	!!$u && !$offset
toDate('s')	re-parse of format('YYYY-MM-DD HH:mm:ss:SSS')	new Date($d)

$x.$localOffset existed only to paper over the host's offset at $d and is
gone. startOf/$set/add in src/index.js need no change: they already
branch on $u, so they now apply the UTC setters to fields that match.

Behaviour change to review

.utcOffset(offset, true) on a UTC-mode instance now keeps the wall clock that
instance displays. It used to keep it only where the host offset happened to
equal the requested one; on any other host the result's format() and
toISOString() disagreed with each other:

// host TZ=America/Vancouver, before this PR
const d = dayjs('2021-02-28 19:40:10').utc().utcOffset(-480, true)
d.format()       // 2021-03-01T03:40:10-08:00  ... which is 11:40:10Z
d.toISOString()  // 2021-03-01T03:40:10.000Z   ... an 8-hour disagreement

Five tests in test/plugin/utc.test.js and one in test/timezone.test.js
encoded that. Each passed dayjs .utc() where it passed moment .utc(true), so
only the moment side kept local time; they now keep it on both sides, which
makes them host-independent, and each gained a format() assertion, since
toISOString() alone could not see the disagreement above.

Tests

• test/plugin/timezone.test.js - the reported instants per zone, plus controls
  at quarter-hour, whole-hour and zero offsets that no gap swallows, plus a sweep
  of every 20 minutes of 2026 across Pacific/Chatham, Australia/Lord_Howe,
  America/St_Johns, Asia/Kolkata and UTC, asserting both the wall clock and
  that valueOf() still equals the instant.
• test/timezone.test.js - the same for .utcOffset() without the timezone
  plugin, including startOf('day') off the shifted wall clock.

Expectations are derived from Intl.DateTimeFormat and from moment, not typed
by hand, so they track the platform's tz database.

The sweeps are the real guard: a fixed instant only reproduces on a host whose
own gap swallows it, but over a year of instants a sweep crosses whatever gap
the host running it has. Against unmodified src/, the new tests fail under
Europe/London and Pacific/Auckland in npm run test-tz, and under all four
hosts of npm run test-tz-plugin; they pass on UTC and America/Whitehorse,
which have no DST.

npm test (all 8 host-timezone runs plus the 100%-line coverage run, 808 tests)
and npm run lint pass.

Refs #2303