## Summary
- Add ISC fixed amount per unit (SUNAT Catalog No. 08, system type 02) support to Peru UBL 2.1 XML generation
- Emit `BaseUnitMeasure` and `PerUnitAmount` at line-level `TaxSubtotal` for ISC tipo 02; emit `TierRange` inside `TaxCategory` for all ISC tipos (01/02/03)
- Compute effective `cbc:Percent` for ISC tipo 02 as `TaxAmount / TaxableAmount × 100` rounded to 5 decimals (SUNAT rules 3102, 3104, 3108)
- Merge ISC variants into a single document-level `TaxSubtotal` (SUNAT rule 3068 — tax code 2000 must not repeat at totals); re-emit `TierRange`/`Percent` at doc level only when all lines share one tipo
- Add tests and XML fixture covering pure tipo 02 and mixed tipo 01+02 invoices

## Context

Peru's ISC tax supports three application systems defined in SUNAT Catalog No. 08:
- **01** — Sistema al Valor (percentage on base) — already supported
- **02** — Aplicación de Monto Fijo (fixed amount per unit) — **this PR**
- **03** — Sistema de Precios de Venta al Público

ISC tipo 02 requires specific UBL nodes per SUNAT specification: `BaseUnitMeasure`, `PerUnitAmount` (at `TaxSubtotal` level) and `TierRange` (inside `TaxCategory`).

### SUNAT validation rules for ISC Percent

For ISC (tax code 2000), SUNAT validates `cbc:Percent` with three rules:
- **3102** — format must be max 3 integer digits and up to 5 decimal digits
- **3104** — value must be different from 0.00 (when `TaxableAmount > 0`)
- **3108** — `Percent/100 × TaxableAmount = TaxAmount` (tolerance ±1)

For ISC tipo 02 (fixed amount), the per-unit amount cannot be placed directly in `Percent` because the validation formula `Percent × TaxableAmount` would fail. Instead, we compute the **effective percentage**: `TaxAmount / TaxableAmount × 100`, which satisfies all three rules.

### SUNAT rule 3068 — single tax code at document totals

When an invoice combines multiple ISC variants (e.g., tipo 01 on one line + tipo 02 on another), they share `TaxScheme/ID = 2000` and SUNAT rejects the document because the same tax code repeats in `cac:TaxTotal` at document totals.

To fix this, `l10n_pe_edi_isc_type` is dropped from the doc-level grouping key so all ISC entries collapse into a single `TaxSubtotal` whose `TaxAmount` and `TaxableAmount` are the sum of every ISC variant. `TierRange`/`Percent` are intentionally omitted at doc level when types are mixed (a merged value cannot represent both tipos). Per-line `TaxSubtotal` nodes still carry their own `TierRange`/`Percent` per tipo. When the invoice uses a single tipo, the type is restored at doc-level render time so existing single-tipo invoices keep their previous output unchanged.

### Line and document base totals

ISC fixed taxes are already excluded from `LineExtensionAmount` and `TaxExclusiveAmount` because `l10n_pe_edi` sets `fixed_taxes_as_allowance_charges = False` (introduced upstream in commit `d07ca711bf7` to fix ICBPER). This PR relies on that behavior; no additional subtraction is performed in the Peru EDI code.

## Legal references
- SUNAT CPE UBL 2.1 specification:
  https://cpe.sunat.gob.pe/sites/default/files/inline-files/guia%2Bxml%2Bnota%20de%20cr%C3%A9dito%2Bversion%202-1%2B1%2B0_0_0%20%282%29.pdf
- SUNAT Catalog No. 08 (ISC System Type):
  https://www.sunat.gob.pe/legislacion/superin/2017/anexoE-245-2017.pdf
- Peru Tax Code Art. 50 — ISC application systems:
  https://www.sunat.gob.pe/legislacion/igv/ley/capitul2.htm
- SUNAT Validation Rules (CPE v2026-02-12):
  AjustesValidacionesCPEv20260212.xlsx — rules 3068, 3102, 3104, 3108

## Example

ISC fixed amount tax configuration (tipo 02 — Aplicación de Monto Fijo):
<img width="1206" height="694" alt="ISC fixed amount tax config" src="https://github.com/user-attachments/assets/87bd23a8-5c5a-42bc-9512-6c19ee45b96d" />

ISC percentage tax configuration (tipo 01 — Sistema al Valor, existing behavior):
<img width="1206" height="694" alt="ISC percentage tax config" src="https://github.com/user-attachments/assets/95e5e914-08d4-4b01-abf3-e5b6159079aa" />

Invoice with ISC fixed amount (tipo 02) — successfully sent to SUNAT:
<img width="1789" height="857" alt="Invoice with ISC fixed amount sent to SUNAT" src="https://github.com/user-attachments/assets/66f89c51-7aac-4a17-a3d3-cc02629fd3f9" />

Invoice with ISC percentage (tipo 01) — backward compatibility confirmed:
<img width="1789" height="857" alt="Invoice with ISC percentage backward compatible" src="https://github.com/user-attachments/assets/9e99387c-b6ed-43aa-8e56-a25e24685172" />

Invoice with ISC — SUNAT validation passed:
<img width="1789" height="857" alt="Credit note with ISC validated by SUNAT" src="https://github.com/user-attachments/assets/ada2016a-aece-404e-b2f1-50193d9eac72" />

## Functional testing (edge cases validated)

| Case | qty | price | ISC/unit | Effective Percent | SUNAT |
|------|-----|-------|----------|-------------------|-------|
| Base case | 50 | 100 | 10.00 | 10.0 | ✅ |
| Different price | 5 | 123 | 2.70 | 2.19512 | ✅ |
| High price | 1 | 530 | 2.70 | 0.50943 | ✅ |
| Very high price | 1 | 99999 | 2.70 | 0.0027 | ✅ |
| Price < ISC | 5 | 2.00 | 10.00 | 500.0 | ✅ |
| Mixed tipo 01+02 (rule 3068) | — | — | — | per line: 17.0 / 2.19512; doc-level: merged into one ISC `TaxSubtotal` (`TierRange`/`Percent` omitted) | ✅ |
| Tipo 02 + solo IGV | — | — | — | 0.50943 | ✅ |