fix: target actual bank.rec.widget (OWL) instead of legacy reconciliation widget — implement rest_debit/rest_credit fields + template extension

dlv_account_reconciliation_rest_amount/README.md

@@ -2,15 +2,15 @@
 
 ## Overview
 
-`dlv_account_reconciliation_rest_amount` extends the standard bank
-reconciliation screen (Accounting → Reconciliation) so the still-open
-("rest") amount of a line is shown in parentheses next to its total.
+`dlv_account_reconciliation_rest_amount` extends the bank reconciliation
+widget (Accounting → Reconciliation, model `bank.rec.widget`) so the still
+outstanding ("rest") amount of a partially matched line is shown in
+parentheses next to its total.
 
-On the reconciliation screen, when a proposed line (the bank movement itself,
-or an added invoice/bill) is only partially used to close the statement line,
-Odoo shows the full amount struck through and the partial amount used above
-it — but not what remains unmatched on that line. This addon adds that
-missing number, e.g.:
+On the reconciliation screen, when an added invoice/bill (a `new_aml` line)
+is only partially used to close the statement line, Odoo shows the full
+source amount struck through below the amount actually used — but not what
+remains unmatched on that line. This addon adds that missing number, e.g.:
 
 ```
 € 1.899,97
@@ -22,19 +22,27 @@ outstanding, without having to search for it elsewhere.
 
 ## How it works
 
-- `static/src/js/reconciliation_rest_amount.js` patches
-  `account.ReconciliationRenderer`'s `ManualLineRenderer.update()`. Before
-  every render, it walks `state.reconciliation_proposition` and, for any line
-  where `partial_amount` differs from `amount`, computes and formats the
-  difference as `line.rest_amount_str`.
-- `static/src/xml/reconciliation_rest_amount.xml` inherits the shared
-  `reconciliation.line.mv_line.amount` template (from `account_accountant`)
-  via xpath and prints `line.rest_amount_str` between parentheses, right
-  after the existing amount.
+- `models/bank_rec_widget_line.py` inherits `bank.rec.widget.line` and adds
+  two computed `Monetary` fields, `rest_debit` and `rest_credit`
+  (`source_balance - balance`, split like the existing `debit`/`credit`
+  fields). Because `bank.rec.widget._compute_lines_widget` automatically
+  serializes every field of `bank.rec.widget.line` to the front-end, these
+  two fields become available client-side as `line.rest_debit` /
+  `line.rest_credit` (each with `.display`, `.value`, `.is_zero`) with no
+  extra plumbing needed.
+- `static/src/xml/reconciliation_rest_amount.xml` extends the OWL template
+  `account_accountant.bank_rec_widget_form_lines_widget`
+  (`t-inherit-mode="extension"`) and prints the rest amount in parentheses
+  next to the struck-through source amount, when it is non-zero.
 
-Because both the bank movement (liquidity line) and any added invoice/bill
-proposition are rendered through the same loop and template, this single
-patch covers both cases.
+### Note on the bank movement itself
+
+Earlier iterations of this addon assumed the bank movement (liquidity) line
+could also show a "rest" amount, based on the legacy (pre-16.4) reconciliation
+widget where that concept existed. In the `bank.rec.widget` model actually
+used on this instance, the liquidity line always reflects the full statement
+line amount — there is no partial/rest concept for it, so nothing is shown
+there. See ROADMAP.md.
 
 ## Dependencies
 
@@ -42,5 +50,6 @@ patch covers both cases.
 
 ## Installation
 
-Standard addon install/update. No configuration or data is added; the change
-is purely visual on the existing reconciliation widget.
+Standard addon install/update (`-u dlv_account_reconciliation_rest_amount`).
+No configuration or data is added; the change is a new computed field plus a
+purely visual template extension on the existing reconciliation widget.