v2: regex parser for Meta UK template + accurate RSU tax attribution

## Context

v1 shipped a Claude Haiku-based extractor that validated only 10/71
backfilled rows. Haiku fumbles the arithmetic on pension salary-sacrifice,
conflates RSU vest with regular earnings, and occasionally misreads YTD
vs this-period columns — so 86% of rows land with validated=false and the
downstream dashboards under-report take-home.

Meta UK uses a stable two-variant template (pre/post 2022-01-31 boundary),
so a regex parser is both faster (ms vs. 30-90s + $0.01-0.05/call) and
more accurate. v2 introduces that parser as the primary path, keeps
Claude as the fallback for non-Meta payslips, and surfaces new fields
the dashboard needs to attribute PAYE between cash salary and RSU vests
correctly.

## This change

### Parser (new)

`payslip_ingest/parsers/meta_uk.py` detects the layout variant by header
presence:

- **Variant A** (pre-2022): vertical Description/This Period/This Year.
  `AE Pension EE` is a positive deduction against a pre-sacrifice gross —
  maps to `pension_employee` for the existing validation formula to hold.
- **Variant B** (post-2022): side-by-side Payments | Deductions | Year to
  Date. `AE Pension EE` is NEGATIVE in Payments (salary sacrifice) — maps
  to `pension_sacrifice` and is already netted into Total Payment.
  `rsu_vest = RSU Tax Offset + RSU Excs Refund` (Meta's template inflates
  Taxable Pay without using a matching offset deduction).

Column boundaries come from the header row's anchor positions; each data
row slices into 3 cells and the last numeric token per cell is the amount.
Anchor misses raise ParserError so the caller falls back to Claude rather
than silently returning bad data.

### New fields

Schema + DB + Claude prompt gain:

- `salary`, `bonus`, `pension_sacrifice` — earnings decomposition for the
  dashboard's bonus-sacrifice visibility and earnings-breakdown chart
- `taxable_pay`, `ytd_tax_paid`, `ytd_taxable_pay`, `ytd_gross` — powers
  the YTD-effective-rate method of attributing cash tax vs RSU tax, which
  is the only method that's accurate month-to-month

All new columns default to 0 / null so v1 rows continue to round-trip.

### Orchestration

processor.py tries `parse_meta_uk(pdftotext(pdf))` first. On success the
result goes straight to the DB — zero Claude tokens spent, extraction in
milliseconds. On ParserError it falls through to ClaudeExtractor as before.
ProcessResult gains an `extractor` field ("meta_uk_regex" | "claude") so
backfill logs show the hit rate.

## Tests

- `test_meta_uk_parser.py` — 11 tests covering variant A, variant B
  (standard + bonus month + bonus-sacrificed month), malformed inputs,
  and end-to-end totals validation for all 4 golden fixtures.
- `test_processor.py` — 2 new tests proving the regex-first short-circuit
  and the Claude fallback on non-Meta inputs.

Fixtures under `tests/fixtures/` are hand-crafted `pdftotext -layout`
emulations — real Meta numbers from the plan's sample payslips for
variant B, synthesized realistic variant A and bonus-sacrificed samples.

0001_initial.py reformat is yapf cleanup touched during the session's
format pass; not a behavior change.

## Test Plan

### Automated

```
$ poetry run pytest
============================= test session starts ==============================
collected 53 items

tests/test_extractor.py .....                                            [  9%]
tests/test_meta_uk_parser.py ...........                                 [ 30%]
tests/test_paperless.py ......                                           [ 41%]
tests/test_processor.py ..............                                   [ 67%]
tests/test_schema.py ....                                                [ 75%]
tests/test_tax_year.py ........                                          [ 90%]
tests/test_webhook.py .....                                              [100%]
============================== 53 passed in 1.66s ==============================

$ poetry run ruff check .
All checks passed!

$ poetry run mypy .
Success: no issues found in 24 source files

$ poetry run yapf --style pyproject.toml --diff --recursive payslip_ingest tests
(no output — all files are yapf-clean)
```

### Manual Verification

Smoke-test the parser against a real Meta payslip PDF on the deploy host:

```
# After 0003 migration applied to prod DB
$ poetry run python -c "
from payslip_ingest.parsers import parse_meta_uk
import subprocess
text = subprocess.check_output(['pdftotext', '-layout', '/path/to/real.pdf', '-']).decode()
p = parse_meta_uk(text)
print(p.model_dump_json(indent=2))
"
```

Expected: JSON with salary/bonus/rsu_vest/pension_sacrifice populated and
`validate_totals(p)` returning True.

## Reproduce locally

1. `cd payslip-ingest && poetry install`
2. `poetry run pytest tests/test_meta_uk_parser.py -v`
3. Expected: 11 tests pass, each fixture validates totals within 2p.

Closes: code-un1

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_processor.py

@@ -1,13 +1,17 @@
 from datetime import date
 from decimal import Decimal
+from pathlib import Path
 from typing import Any
 from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 
+from payslip_ingest import processor
 from payslip_ingest.processor import process_document
 from payslip_ingest.schema import ExtractedPayslip
 
+FIXTURES = Path(__file__).parent / "fixtures"
+
 
 def _sample_extraction() -> ExtractedPayslip:
     return ExtractedPayslip(
@@ -164,3 +168,37 @@ async def test_process_document_flags_validation_failure(paperless: AsyncMock,
     assert result.status == "inserted"
     assert result.validated is False
     assert factory.used[1].added[0].validated is False
+
+
+async def test_regex_parser_short_circuits_claude(paperless: AsyncMock, extractor: AsyncMock,
+                                                  monkeypatch: pytest.MonkeyPatch) -> None:
+    """When pdftotext output matches the Meta template, Claude must not run."""
+    meta_text = (FIXTURES / "meta_uk_2026_02.txt").read_text(encoding="utf-8")
+    monkeypatch.setattr(processor, "_pdftotext", lambda _: meta_text)
+
+    factory = _SessionFactory([_FakeSession(existing_ids=[]), _FakeSession(existing_ids=[])])
+    result = await process_document(42, factory, paperless, extractor)
+
+    assert result.status == "inserted"
+    assert result.validated is True
+    assert result.extractor == "meta_uk_regex"
+    extractor.extract.assert_not_called()
+    # Salary / bonus / pension_sacrifice from the regex parser should land on the row.
+    row = factory.used[1].added[0]
+    assert row.salary == Decimal("10003.33")
+    assert row.pension_sacrifice == Decimal("600.20")
+    assert row.rsu_vest == Decimal("30479.76")
+    assert row.taxable_pay == Decimal("72096.92")
+
+
+async def test_regex_miss_falls_back_to_claude(paperless: AsyncMock, extractor: AsyncMock,
+                                               monkeypatch: pytest.MonkeyPatch) -> None:
+    """When pdftotext output doesn't match Meta, Claude is invoked."""
+    monkeypatch.setattr(processor, "_pdftotext", lambda _: "Some other employer's payslip\n")
+
+    factory = _SessionFactory([_FakeSession(existing_ids=[]), _FakeSession(existing_ids=[])])
+    result = await process_document(42, factory, paperless, extractor)
+
+    assert result.status == "inserted"
+    assert result.extractor == "claude"
+    extractor.extract.assert_awaited_once()