docs: recast NEWS around the first public release

NEWS described changes relative to states no user had ever seen --
'Breaking: corpus schema_version 4', 'the package now requires...' --
across the whole pre-release development. To someone deciding whether to
depend on this, that reads as instability.

0.3.0 is written as an announcement: what it covers, the verbs, that
reading the corpus now works out of the box, four things to know before a
first query, and the known limits. The 0.2.0 changelog is kept verbatim.
The 0.1.0 development log is dropped; that history is in git.

cog_explain() now documents what provenance actually holds, since the
README points readers at it -- in particular why series_break_refs and
corpus_break_refs are separate fields rather than one list.

man/cog_explain.Rd

@@ -21,3 +21,33 @@ Prints the structured provenance attached to a tibble returned by any
 `cog_*` verb, or returns it as a list for downstream use (MCP tools,
 dashboards, JSON export).
 }
+\section{Two kinds of series break}{
+
+Catalogued breaks reach you without being asked for, in two disjoint
+fields, because a caveat about one series and a caveat about the whole
+corpus are different claims:
+
+* **`series_break_refs`** — breaks matched against the item codes actually
+  present in this result. A break in one code you queried.
+* **`corpus_break_refs`** — breaks catalogued with `fin_code = "ALL"`,
+  which are statements about the corpus rather than about any one code:
+  dollar precision across the 1976/1977 boundary (`SB085`), imputation
+  exclusion from FY2002 (`SB087`), the FY2012 dense-to-sparse
+  representation change (`SB194`), and the FY2017 government-identifier
+  change (`SB086`). These are selected on the break-year window alone.
+
+`SB194` is the one most likely to matter: a query spanning FY2011 to FY2012
+crosses the boundary where an absent cell stops meaning "Census published
+$0" and starts meaning "not reported".
+}
+
+\section{Other provenance blocks}{
+
+`transformations$units_conversion` records the `$1,000s`-to-dollars
+multiply that every amount column has already had applied.
+`transformations$per_capita` records the population denominator and its
+year range. `coverage` and `coverage_mode` appear on multi-government
+results (see [cog_geographic_rollup()]). `completion` appears when
+`complete = TRUE`. `balance_caveats` appears on [cog_balances()] results.
+}
+