fix(date-field): show PM times correctly in 12-hour display

[interacsean]

Closes tailor-inc/platform-planning#2039

Problem

With granularity at hour/minute/second and a 12-hour cycle (hourCycle={12} or a 12-hour locale like en-US), DateField / DatePicker / DateRangePicker showed afternoon values as e.g. "18:00 AM", and editing any segment emitted the value 12 hours early (06:xx).

fieldsFromValue() copied the 0–23 hour straight into the segments and never set dayPeriod, while composeValue() reads 12-hour segments as hour % 12 + (pm ? 12 : 0).

Fix

• fieldsFromValue(v, is12) stores the hour as 1–12 + dayPeriod in 12-hour mode (midnight → 12 AM, noon → 12 PM). Used for the initial value, external controlled syncs, and the anchor, so ArrowUp on an empty hour after noon no longer seeds an out-of-range 18.
• If the hour cycle flips on a mounted field (hourCycle prop or a locale change that flips the default), the stored segments are re-encoded during render. The composed value is unchanged, so onChange doesn't fire. Half-typed hours convert too.
• Docs: added the missing granularity / hourCycle rows to the DatePicker props table.

Verification

• 11 new regression tests across date-field.test.tsx and date-range-picker.test.tsx. Against the old hook, 9 fail (the 00:00 minute-edit case passes there by coincidence, since 0 % 12 is AM); without the cycle-change conversion, exactly the 3 switch tests fail.
• Browser (vite-app showcase, temporary demo not committed): 18:00 → 06:00 PM; minute ArrowUp → 18:01; toggling 12↔24 shows 18:01 / 06:01 PM; a/p on AM/PM → 06:01 / 18:01.
• Local: fmt, lint, type-check, test, build, docs:check all pass.

🤖 Generated with Claude Code

[interacsean]

/review

[github-actions]

✅ Code Review completed successfully!

Code review complete for PR #586: reviewed changed package implementation under packages/core, checked prior PR review threads/comments, and found no actionable High/Medium/Low package issues to flag. No GitHub write was needed because there were no inline findings to post.

Generated by Code Review for #586

[github-actions]

Code Metrics Report

	main (529b3ac)	#586 (1bc7d88)	+/-
Coverage	87.6%	87.9%	+0.3%
Test Execution Time	2m14s	2m19s	+5s

Details

  |                     | main (529b3ac) | #586 (1bc7d88) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          87.6% |          87.9% | +0.3% |
  |   Files             |            206 |            206 |     0 |
  |   Lines             |           6090 |           6097 |    +7 |
+ |   Covered           |           5335 |           5364 |   +29 |
- | Test Execution Time |          2m14s |          2m19s |   +5s |

Code coverage of files in pull request scope (89.5% → 94.6%, patch 93.3%)

Files	Coverage	+/-	Patch Coverage	Status
packages/core/src/components/date-field/date-field.tsx	100.0%	0.0%	-	modified
packages/core/src/components/date-field/date-input-group.tsx	84.4%	+2.9%	-	affected
packages/core/src/components/date-field/date-range-picker.tsx	97.7%	0.0%	-	modified
packages/core/src/components/date-field/use-date-field-state.ts	96.8%	+10.3%	93.3%	modified

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Reported by octocov