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.

NEWS.md

@@ -19,6 +19,19 @@
 * `cog_categories()` advertises `"All Categories"` for the expenditure and
   revenue vocabularies, so the reserved value is discoverable.
 
+* Coverage signposting (see "Signposting now catches partially-suppressed
+  categories" below) now also works in `category = "All Categories"` mode.
+  The recipe-suggestion candidate query used to be scoped by `category`,
+  which is never a match for the reserved `"All Categories"` value, so
+  `provenance$suggestions` always came back empty there — the one mode whose
+  whole point is "you cannot sum the wrong scope" was silently unable to
+  signal a wrong scope. The candidate query is now scoped by the concept's
+  subtype allowlist instead, symmetric with how `.build_verb_sql()` itself
+  scopes the summed total: Los Angeles County FY2011, `category = "All
+  Categories"` still excludes $271,589,000 of aggregate-published Public
+  Welfare (`E68`), but now names `recipe = "welfare_cash_e68_wide"` to
+  recover it instead of reporting zero suggestions.
+
 ## Documentation
 
 * `cog_geographic_rollup()` and `cog_peer_compare()` now document that