fix: stabilize flaky CI (stage-exec-test (from-0, paral)

[erigon-copilot]

Title

db/state: invalidate BranchCache entries on Flush to fix stale trie reads

Root Cause

The BranchCache introduced in #21380 (State Cache Consolidation) is an
aggregator-scoped commitment-branch cache that lives across batch boundaries.
SharedDomains.Commit() correctly updates the cache after a successful commit,
but the Flush() path — used by the integration stage_exec from-0 loop — did
not invalidate or update the cache at all.

In the from-0 execution loop (cmd/integration/commands/stages.go:802–837):

1. Batch N reads a commitment branch from MDBX → cached in BranchCache
2. Batch N updates the branch via execution → written to sd.mem
3. Flush() writes sd.mem to the MDBX tx (but does NOT touch BranchCache)
4. ClearRam() clears sd.mem (but NOT the BranchCache)
5. tx.Commit() persists to disk
6. Batch N+1 reads the same branch → BranchCache returns the stale value from
   step 1, bypassing MDBX which has the correct value from step 3

This produced wrong trie roots because the trie computation used stale
intermediate branch nodes instead of the freshly committed ones.

Fix

Flush() now uses the existing flushMemWithCallback path to invalidate every
commitment-domain key it writes. Subsequent reads fall through the (now-empty)
cache to MDBX and get the correct value.

The invalidation-only approach (vs. populating the cache like Commit() does)
is deliberately conservative: Flush() callers own the commit, so we cannot
guarantee the tx will actually be committed. Invalidation is safe regardless of
commit outcome.

Verification

• TestBranchCacheFlushInvalidatesStaleEntries — new unit test that reproduces
  the exact stale-cache scenario. Confirmed: FAILS without the fix (returns
  stale v1), PASSES with the fix (returns correct v2).
• TestBranchCacheCommitRefreshesAfterReadThrough — existing test, still passes.
• TestFromZero_GenesisAllocPreservedAfterResetReExec — passes (serial + parallel).
• All db/state/execctx tests pass with -count=3 -race.
• All execution/commitment tests pass with -count=3 -race.
• make lint clean, make erigon integration build clean.