fix(timezone): half-hour DST shift for Australia/Lord_Howe

[spokodev]

Australia/Lord_Howe uses a 30-minute DST shift, but spacetime reports it as a full hour, so every instant in the DST half of the year is 30 minutes off.

Repro

const spacetime = require('spacetime')
const tz = 'Australia/Lord_Howe'

// January = southern summer = DST on
spacetime(new Date('2026-01-15T00:00:00Z').getTime(), tz).offset() // 690  (+11:30)

// native Intl oracle
new Intl.DateTimeFormat('en-US', { timeZone: tz, timeZoneName: 'longOffset' })
  .formatToParts(new Date('2026-01-15T00:00:00Z'))
  .find(p => p.type === 'timeZoneName').value // 'GMT+11:00'

ACTUAL: +11:30 (690 min). EXPECTED: +11:00 (660 min).

Root cause

In src/timezone/index.js, the "other-season" offset is derived by hardcoding a 1-hour DST shift (winter = summer - 1 north, found.offset + 1 south). Lord Howe is the only IANA zone with a 30-minute DST shift (standard +10:30, DST +11:00). Its zonefile stores the standard offset 10.5; adding a full hour yields +11:30 instead of +11:00 for the summer (DST) side. This affects offset(), formatting, and UTC-to-wall-clock conversion for the whole DST half of the year.

The July / standard side was already correct (+10:30), because it reads the stored offset directly.

Fix

Derive the shift per zone and special-case the single half-hour zone:

const dstShift = tz === 'australia/lord_howe' ? 0.5 : 1

Then use dstShift in place of the hardcoded 1 for both hemispheres.

Tests

Added a case to test/dst-south.test.js that asserts offset() against the native Intl longOffset oracle for both seasons: January +11:00 and July +10:30.

Red baseline: with the source change reverted, the January assertion fails (690 vs 660); July already passes. With the fix, both pass.

Full suite (tape): 6080 tests. Before the fix 12 failed (11 pre-existing America/Denver minute-boundary cases in test/dst-sneak.test.js, plus the new Lord Howe case). After the fix 11 fail, all of them the same pre-existing Denver cases, which this change does not touch.

[spencermountain]

thank you!!