fix: scope all-categories suggestion candidates by subtype, not category (finding 6)

.build_suggestions()'s recipe-candidate sub-select was keyed on
`WHERE category IN (<category>)`. The reserved pseudo-category
"All Categories" is never itself a row in summary_categories.category, so
in all-categories mode `candidates` always came back empty and coverage
signposting (uscogdata#9) was structurally impossible for the one mode
whose entire premise is "you cannot sum the wrong scope" -- measured on
Los Angeles County FY2011: category = "Public Welfare" reports 2
suggestions (incl. $271,589,000 excluded E68), category = "All Categories"
reported 0, silently losing that same signal.

Apply the branch's own design principle: the concept boundary is subtype,
not category. .build_suggestions() now accepts all_categories/subtype_col/
subtype_scope (all optional, default off, so no other caller's behaviour
changes) and, when all-categories mode is active, scopes the candidate
sub-select by `<subtype_col> IN (<subtype_scope>)` instead -- symmetric
with .build_verb_sql()'s own WHERE predicate. The M/L recipe exclusion and
the is.null(category) early return are unchanged.

After the fix, LA County FY2011 "All Categories" reports 5 suggestions,
including welfare_cash_e68_wide for the exact $271,589,000 gap.

Adds two covering tests to test-all-categories.R using the bundled fixture
(AL state gov, FY2011, "Corrections"): one end-to-end (per-category and
all-categories both signpost the same recipe) and one direct on
.build_suggestions() proving the subtype-vs-category branch is what
changes the query. Updates the 0.2.0 NEWS entry.

R/spending.R

@@ -442,7 +442,10 @@ cog_spending <- function(govid, years, category = NULL,
     suggestions <- .build_suggestions(con, govid, years, category,
                                        direct_leg_result,
                                        resolved$basis, flow_prefixes,
-                                       .select_long_view(view_base, resolved$basis))
+                                       .select_long_view(view_base, resolved$basis),
+                                       all_categories = all_categories,
+                                       subtype_col = subtype_col,
+                                       subtype_scope = subtype_scope)
   }
 
   # C1(b): when expenditure_concept = "total", flag any row where the IG