0.2.43: historical NAV backfill without touching current holdings

The batch history import now also records each quarter's NAV in the
fund's valuation history: the old file's HLD rows are matched by issuer
and security name against the book as it exists today, matched rows
write that quarter's valuations, unmatched rows are counted and
reported, and nothing outside the round is created or modified. A
manually signed quarter is never overwritten.

The single-file wizard automatically takes the same history-only path
when the file is older than the fund's newest round. Previously that
import would regress position cost basis to the old file's values and
resurrect since-exited positions, corrupting the fund's Invested total.

frontend/public/sw.js

@@ -3,7 +3,7 @@
 // - content-hashed /assets/* are cache-first (immutable, safe forever)
 // - /api/* is never cached
 // Bump CACHE on each release so old entries are purged.
-const CACHE = 'ten31-portal-0.2.42'
+const CACHE = 'ten31-portal-0.2.43'
 
 self.addEventListener('install', () => self.skipWaiting())