fix(salary): pair a bare Count/Index column pair instead of splitting it into two categories (#470)

The pairing loop required a non-empty derived category name on both
sides, but a header with no category word - "Count" + "Index", Danish
"Antal" + "Lønindeks" - strips to an empty name, so the simplest layout
the README advertises ("auto-pairs count/index columns") came out as two
unrelated standalone categories:

  {"count": {"count": 500}, "index": {"index": 108.5}}

salary_lookup then rendered a "Count  500  N/A*" row above an
"Index  -  108.5" row, and its footnote read the N/A* as "too few
employees to publish (privacy)" - a false statement about a company whose
headcount is in the file, shown during /apply's salary step. Any suffix
("Antal alle") made pairing work, which is why the shipped tests, all
suffixed, never saw it.

Pair on equal derived names, empty included, and give the nameless pair
the README's top-level category name (all_employees). A bare "Antal" with
no bare index column still stays a standalone count; named pairs
alongside are untouched.

Four new cases in test_convert_salary_excel.py, one of them rendering the
converter's output through salary_lookup.format_entry; all four fail
against the old pairing rule.

tools/convert_salary_excel.py

@@ -52,6 +52,9 @@ COMPOUND_PATTERNS = {"antal", "indeks", "løn", "gennemsnit", "medarbejdere"}
 # data. They are dropped at classification so they are not mistaken for a salary
 # category. Matched as whole tokens only, like other pattern sets.
 ID_PATTERNS = {"id", "personnummer"}
+# Category name for a count/index pair whose headers carry no category word at
+# all ("Count" + "Index"). Matches the top-level category in README_SALARY_TOOL.md.
+DEFAULT_CATEGORY = "all_employees"
 
 
 def parse_numeric_cell(value):
@@ -216,7 +219,14 @@ def parse_sheet(ws, sheet_label=None):
         else:
             untyped_cols.append((col_idx, col_header))
 
-    # Pair count/index columns by matching category name
+    # Pair count/index columns by matching category name. A bare "Count" /
+    # "Index" pair (Danish "Antal" / "Lønindeks") strips to an empty name on
+    # both sides - the single-category layout the README's "auto-pairs
+    # count/index columns" line describes. It is still one pair, so it gets
+    # the README's default category name instead of being emitted as two
+    # unrelated standalone columns: salary_lookup renders that split as a
+    # count row whose index reads "N/A*", i.e. "too few employees to publish
+    # (privacy)", about a company with a published headcount.
     categories = []
     used_counts = set()
     used_indexes = set()
@@ -225,8 +235,8 @@ def parse_sheet(ws, sheet_label=None):
         for ii, (i_idx, i_header, i_cat) in enumerate(index_cols):
             if ii in used_indexes:
                 continue
-            if c_cat and i_cat and c_cat == i_cat:
-                cat_name = c_cat.replace(" ", "_").replace("-", "_")
+            if c_cat == i_cat:
+                cat_name = (c_cat or DEFAULT_CATEGORY).replace(" ", "_").replace("-", "_")
                 categories.append({
                     "name": cat_name,
                     "count_col": c_idx,