Addresses review finding C1.
Before this commit, flushStem in generateBinTrieStems used
builder.encode() to overwrite the on-disk stem blob unconditionally.
When a crash+restart interrupted generation mid-stem (e.g., at offset 3
of stemA), the resume iterator positioned at stemA||3, the builder
accumulated only offsets 3+, and flushStem overwrote the disk blob with
a partial result — silently losing offsets 0, 1, 2 that were written in
the prior pass.
Fix: make flushStem a read-modify-write. It now reads the existing
on-disk stem blob (if any), converts the builder's accumulated offsets
to []stemOffsetValue via a new toOffsetValues() helper, and merges them
via the existing mergeStemBlob function. The merge semantics are
"builder values win" — new offsets overwrite their existing counterparts,
and gaps are filled from the prior blob. This makes the RMW idempotent
across resume cycles: the same stem can be re-walked from any midpoint
and the final disk blob always contains the union of all passes.
New helper: stemBuilder.toOffsetValues() converts the builder's
populated bitmap entries into a []stemOffsetValue slice suitable for
mergeStemBlob. ~20 LOC in stem_blob.go.
Tests:
* TestBintrieGeneratorResumeMidStem — pre-seeds disk with a partial
stem (offsets 0, 1), resumes generator at offset 1, asserts all
offsets survive including the pre-seeded offset 0. Before the fix
this test fails with "BasicData lost after mid-stem resume".
* TestBintrieGeneratorResumeStemBoundary — renamed from the original
TestBintrieGeneratorResume, unchanged behavior (stem-boundary
resume).
Adds generateBinTrieStems, the bintrie analogue of generateAccounts. It
opens the bintrie via a sha256-aware bintrieDiskStore (the merkle disk
store would always fail root validation against a binary node), iterates
all leaves with binaryNodeIterator, aggregates them into per-stem
builders, and emits one stem blob per stem boundary.
Resume support is structural: ctx.marker is fed straight to the trie's
NodeIterator, which uses binaryNodeIterator.seek (Commit 1) to position
on the first leaf >= marker. Range proofs are deliberately skipped — the
bintrie's Prove path is unimplemented and an iteration-only generation
cycle is acceptable for a one-time startup cost.
A bintrieGeneratorContext mirrors generatorContext but is much smaller:
no holdable iterators (we walk the trie, not the existing flat state)
and no two-tier marker (the bintrie key space is unified). checkAndFlushBin
journals progress as a single 32-byte (stem || offset) key so resume
can pick up mid-stem.
generator.run dispatches on codec type so callers see a uniform
lifecycle whether the underlying scheme is merkle or bintrie.
