Record O9 decision: expense materials at purchase (periodic inventory)

Owner decision: this is a service business that uses materials to deliver a service
rather than selling inventory, so powder/consumables are expensed at purchase (bill
to a COGS/expense account) and inventory is not capitalized on the Balance Sheet.

No code change required — the Balance Sheet already behaves this way, and the
perpetual consumption-COGS path (O6) is opt-in via item account mappings (set only
via CSV import) and stays dormant under this policy. Documents the double-count
footgun (do not both expense at purchase and map item COGS/Inventory accounts) and
locks the periodic choice into the ledger-refactor plan's Phase 5.

All accounting audit findings O1-O9 now resolved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ACCOUNTING_AUDIT.md

@@ -193,19 +193,24 @@ parallel with the live postings. The durable fix is to make **every** financial
 lines and drive all reports/recompute from those lines alone (single source of truth) — O8's write-off already
 does exactly this, which is why it was the cleanest. Worth considering before the ledger grows further.
 
-### O9 — Balance Sheet does not capitalize/relieve inventory (periodic vs perpetual) — **OPEN, needs policy decision**
-- Surfaced while fixing O6. The **Trial Balance** computes an inventory asset as opening + purchase bill-lines
-  − consumption (perpetual), but the **Balance Sheet** `ComputeBalance` does **not** add inventory purchase
-  bill-lines to the asset and instead expenses all bill costs through retained earnings (periodic). So the BS
-  inventory line ≈ opening balance only, and BS vs TB disagree on inventory. O6's consumption relief was
-  therefore intentionally **not** applied to the BS (doing so alone would drive inventory negative / unbalance
-  the sheet).
-- **Impact:** Balance Sheet inventory asset is understated and COGS timing differs between BS (periodic) and
-  P&L/TB (now perpetual for consumption). Pre-existing; not a trial-balance imbalance.
-- **Fix direction:** decide on an inventory accounting policy (perpetual vs periodic) and make the Balance
-  Sheet consistent with the Trial Balance / P&L. Best done as part of the JournalEntry single-source refactor.
+### O9 — Inventory accounting policy — **RESOLVED (policy decision: expense at purchase / periodic)**
+- **Decision (owner, 2026-06-19):** materials (powder, consumables) are **expensed at purchase**, not
+  capitalized as inventory-for-resale — this is a service business that *uses* materials to deliver a
+  service, it does not sell inventory. So the **periodic** model is correct.
+- **Implication — no code change needed:**
+  - The Balance Sheet correctly does **not** capitalize inventory (cost hits the P&L at purchase via the
+    bill line categorized to a COGS/expense account). The earlier decision to leave the BS untouched stands.
+  - Purchase receiving creates a `Purchase`-type `InventoryTransaction` that posts **no** GL — correct.
+  - The **perpetual** consumption-COGS path (O6: `DR COGS / CR Inventory` on JobUsage/Waste) is **opt-in**:
+    it only fires when an item has *both* `CogsAccountId` and `InventoryAccountId`, which are set **only via
+    CSV import** (never by normal item creation). Under this policy those mappings should be left empty, so
+    the path stays dormant. O6's recompute fix remains correct and harmless when unused.
+- **⚠ Footgun to avoid:** do **not** both expense powder at purchase (bill → COGS account) *and* map an
+  item's `CogsAccountId` + `InventoryAccountId` — that would record the cost twice (once at purchase, once at
+  consumption). Keep item COGS/Inventory account mappings empty under the expense-at-purchase policy.
 
 ## Status
-Findings **O1–O8 + the read-path sweep are resolved** on `dev`. **O9 is newly identified and OPEN** (inventory
-capitalization policy — needs a decision, recommend folding into the JournalEntry single-source refactor).
+**All findings O1–O9 + the read-path sweep are resolved** on `dev` (O9 by policy decision — expense at
+purchase — needing no code change). The optional structural follow-up is the JournalEntry single-source
+refactor (`docs/ACCOUNTING_LEDGER_REFACTOR.md`), which would prevent the O2/O6/O7/O8 bug class from recurring.
 Original audit numbering #1–3/#5/#6/#8 remains unrecoverable (see top). Nothing merged to `master` yet.