Fix the self-contradictory due_offset_days description

[os-warren]

Fixes #23

Documentation-only fix. Reading A is adjudicated (see the PM comment on #23)
and shipped in src/functions/period.ts (#1): due_offset_days counts days
from the anchor day, and 0 means the anchor day itself. That arithmetic is
untouched here — the bug was the field's own description contradicting it
("5" with a monthly period anchored to period start = due on the 5th, which
is off by one under reading A).

What changed

1. src/objects/duty.object.ts — rewrote due_offset_days.description with a
   worked example on each anchor: period_start — 0 = first day, 4 =
   fifth day; period_end — 0 = last day, -3 = three days before the
   last. Also changed the label from Offset (days) to
   Offset (days, 0 = anchor day) so the zero-point reads without opening the
   description tooltip.
2. src/objects/catalog-item.object.ts — the same field, the same label and
   description. It had no description at all before this PR; it now matches
   duty.object.ts verbatim, which matters because Catalog instantiation — apply a position's duty catalog to a person #5's catalog
   instantiation copies cadence fields from the catalog item onto the duty —
   a mismatch between the two would have replicated a contradiction onto
   every duty created from a catalog item.
3. docs/product/data-model.md — the due_anchor + due_offset_days
   paragraph gets the same worked examples.

Explicitly not touched

src/functions/period.ts and test/period.test.ts — the arithmetic is
correct and adjudicated; this PR only fixes the prose that misdescribed it.
objectstack.config.ts was not touched either.

Gates (all green, at a9d15af)

pnpm validate    # ✓ Validation passed (394ms) — 5 Objects, 57 Fields
pnpm typecheck   # tsc --noEmit — no output, exit 0
pnpm test        # Test Files 5 passed (5) · Tests 175 passed (175)
pnpm build       # ✓ Build complete (589ms) — dist/objectstack.json (63.1 KB)

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