Fix egg-farm spec gaps: hen-day math, egg unit conversion, regrade path #2

Doc-only corrections to specs/product/specs.md.

1. Hen-day production % — was total eggs / current live birds (inflates rate, can exceed 100% on period reports). Now divides by point-in-time hen-days from the bird ledger, restricted to laying females; period rate sums daily hen-days.

2. Egg unit conversion — eggs stored individually but sold in dozen/tray/carton/case with no factor defined. Adds farm-configurable egg_unit_conversions + immutable base_unit_factor snapshot on sales_order_items; blocks confirmation on units with no conversion.

3. Grading/packing — adds Phase 1 grade-at-collection assumption + reserved egg_grade_transfers path (paired grade_in/grade_out movements, net-zero, traceability preserved) for Phase 1.5.

Scaffolding impact: none. All three are spec additions for unbuilt features; src/ walking-skeleton untouched.

Migrated from Gitea PR #2 (closed). Original: https://gitea.mforcelabs.com/mforce/cluckwork/pulls/2. The code is in main's commit history.