docs: record loan decisions — separate ledger, 50/50 fixed, $4,000 receivable

The loan is a separate ledger, not a settlement context: a contribution must
never be able to settle a dinner.

The share is fixed at 50%, not derived from actual payments. During Sonu's leave
the obligation did not change, only the payment did - a percentage-of-actual
model would silently redefine her share as 30% and make the shortfall vanish. So
the model needs an expected schedule alongside actual contributions, with the
difference as a tracked receivable. Currently $4,000.00 over Jul 2025 - Jun 2026.

On interest: recorded the mechanics (it is debited to the loan and repaid as part
of the balance - the reconciliation is exact) alongside the counter-argument that
$16,523.64 left and bought nothing, which is what an expense is. Recommends
keeping it as spend with a fixed/discretionary grouping to address the real
concern, but flags it as a judgement call rather than settling it.

docs/shared-expenses-design.md

@@ -125,16 +125,20 @@ attribution layer over a real transaction rather than a parallel record of it:
 - A payment with no matching transaction (cash, or an account not imported)
   stays as a manual row — the model must tolerate that
 
-### 3. The shared loan
+### 3. The shared loan — a separate ledger
 
-The loan needs a co-ownership share separate from expense splitting, because it
-is not a periodic shared expense — it is a jointly funded asset.
+Not a settlement context. The loan is a jointly funded asset with its own
+obligation, and mixing it with expense settlement would let a contribution
+accidentally settle a dinner.
 
 - `emi` credits are recognised as **contributions**, not generic transfers
-- Contribution share drives how `loan_interest` is attributed to spend
-- Equity (principal) accrues per participant
+- A **contribution schedule** states what is owed per period (50% of the
+  repayment), independent of what was actually paid
+- The running difference is a **receivable** — currently $4,000.00
 
-**This is the piece I am least sure about** — see open questions.
+The schedule matters: during Sonu's leave the obligation did not change, only the
+payment did. A percentage-of-actual model would silently redefine her share as
+30% and make the shortfall disappear.
 
 ---
 
@@ -153,20 +157,69 @@ is not a periodic shared expense — it is a jointly funded asset.
 
 ---
 
+## Decisions taken (2026-07-26)
+
+### The loan is separate from shared expenses
+
+Different obligations, different rhythms, different nature: one funds an asset,
+the other funds consumption. They do not share a settlement context and a
+contribution is never a settlement.
+
+### The share is 50/50, fixed — with the shortfall tracked
+
+Not derived from actual payments, which fluctuate. Sonu's obligation is half the
+repayment; the difference between obligation and actual is a **receivable**, and
+it is the interesting number.
+
+Over 2025-07-01 → 2026-06-30:
+
+| | |
+|---|---:|
+| Repayments | $63,500.00 |
+| Sonu's 50% obligation | $31,750.00 |
+| Actually contributed (26 payments) | $27,750.00 |
+| **Shortfall** | **$4,000.00** |
+
+She never missed a fortnight; the rate changed:
+
+| Rate | Payments | Period |
+|---|---:|---|
+| $1,250 | 15 | Aug 2025 – Feb 2026 (the correct 50%) |
+| $1,000 | 3 | Jul 2025 (pre-adjustment) |
+| $750 | 8 | Mar – Jun 2026 (leave) |
+
+So the model needs a **contribution schedule** (expected per period) alongside
+actual contributions, with the running difference as a tracked balance. A flat
+percentage cannot express "obligation unchanged, payment temporarily reduced,
+difference owed".
+
+### Interest: recommended as expense, pending final call
+
+The mechanics are as described — interest is debited to the loan and repayments
+pay down the combined balance. Reconciles exactly:
+
+    134:  31,000.00 − 8,553.27             = 22,446.73  = balance reduction
+    133:  32,500.00 − 7,970.37 − 2,849.00  = 21,680.63  = balance reduction
+
+But mechanics are not the same as economics. Over 12 months $63,500 of cash left
+and debt fell by $44,127.36. The $16,523.64 difference bought nothing and is not
+recoverable — that is an expense by definition. Excluding it leaves the balance
+sheet unable to reconcile cash out against equity gained, and understates annual
+cost by ~10%.
+
+The legitimate concern is that interest is **non-discretionary**. The answer to
+that is a fixed-commitments grouping alongside rent, insurance and utilities —
+a presentation change, not an exclusion.
+
+**Recommendation: keep `loan_interest` as spend, add a fixed/discretionary
+split.** Flagged rather than settled: it is a judgement about what "spend" means
+in your own reporting.
+
 ## Open questions
 
-1. **Should loan interest be split?** If Sonu funds 31% of repayments, is 31% of
-   the $16,523.64 interest her expense — or is the loan simply yours with her
-   contributing, and the interest all yours? This is a decision about the
-   arrangement, not a technical one.
-
-2. **Does equity need tracking per person?** If Sonu accrues a share of the
-   principal, that is a balance-sheet item the app has no concept of. It may
-   belong in the future net-worth view rather than here.
-
-3. **Is the contribution share fixed or derived?** Derived from actual `emi`
-   payments it fluctuates every fortnight. Fixed, it needs stating and
-   maintaining. Derived is more honest; fixed is more stable for analytics.
+1. **Does equity need tracking per person?** If Sonu accrues a share of the
+   principal, that is a balance-sheet item the app has no concept of. Probably
+   belongs in the net-worth view rather than here.
 
 4. **Attribution of forwarded payments.** `mummy` in the description reliably
    marks Molina's money in all six known cases, but it is a description match on