specs: §10.2/§10.4/§10.5 drift found while building #720 #737

Three inaccuracies in specs/product/specs.md, all found while building #720 (PR #734) and deliberately left out of that slice to keep it scoped.

1. §10.5 lists a column that does not exist

line_total_cents is in the canonical sales_order_items schema. There is no such column — LineTotal is computed on the entity and explicitly EF-ignored (SalesOrderConfiguration.cs, builder.Ignore(i => i.LineTotal)).

2. §10.5 omits a column that does exist

egg_grade_id is a real, required, FK-constrained column on SalesOrderItems and is absent from the canonical list.

3. §10.4 and §10.2 now collide with #720's vocabulary

• §10.4 reserves discount_cents on sales_orders for an entered, order-level amount. Sales: snapshot the list price on the order line (discount foundation) #720 introduced a derived, per-line discount under the same English word. Sales: snapshot the list price on the order line (discount foundation) #720's glossary disambiguates them; the spec does not.
• §10.2 states "Cluckwork does not derive or enforce a discount from another product." Sales: snapshot the list price on the order line (discount foundation) #720 derives a discount from the same product's own list price. Not strictly contradicted, but it reads as though the system derives no discounts at all, which is no longer true. Future: coupon codes as the discount mechanism, instead of typed prices #730 (coupon codes) independently flags this sentence as needing deliberate amendment.

Why it was not fixed in #720

#720 added its own column to §10.5 and stopped. Correcting surrounding drift would have widened a slice already carrying a migration, a UI change and three locales — and the §10.2/§10.4 wording needs a vocabulary decision, not just a correction.

Suggested scope

Fix 1 and 2 as plain corrections. Handle 3 alongside #730's §4.3/§10 amendment, since that slice must touch the same prose and will otherwise write on top of it.