fix(sqlite_writer): skip index cells that exceed a single page

[jjoos]

Problem

write_index_btree() crashes with SIGBUS when indexing large repositories (observed on a Ruby codebase with ~215K nodes, specifically the HackerOne core monolith with ~67K functions).

The crash occurs in the page-buffer writer when an index cell's payload (qualified_name + file_path + properties) exceeds the SQLite page size. The code flushes the current page and assumes the fresh page will always accept the cell — but oversized cells still fail pb_cell_fits() after the flush. The subsequent pb_add_cell() causes a content_offset underflow, corrupting the page header and triggering SIGBUS on the next write.

Reproduction

Index any repository with functions whose qualified names + file paths exceed ~4000 bytes combined (the SQLite page size minus overhead). A Ruby monolith with deeply nested namespaces and long file paths reliably triggers this.

codebase-memory-mcp index --path /path/to/large-ruby-repo
# → SIGBUS in write_index_btree at sqlite_writer.c:1218

Fix

After pb_promote_and_flush(), re-check pb_cell_fits(). If the cell still doesn't fit on an empty page, continue past it. Index entries whose keys exceed a full page cannot be stored in a leaf and are silently dropped — the rest of the index is correctly preserved.

This is the index-btree counterpart to PR #175 (record overflow pages), which fixed the same shape of bug for the record-btree writer. The record side uses overflow pages; the index side has no overflow mechanism, so skipping is the correct behavior.

Changes

• internal/cbm/sqlite_writer.c: 10 lines added, 3 removed in write_index_btree().

Testing

• Verified by indexing a 215K-node Ruby repo that previously crashed — completes successfully with all other nodes indexed.
• The skipped cells are edge cases (extremely long qualified names) that don't affect query correctness for normal usage.

Related

• Closes the index-cell side of Stack overflow crash in autoindex_thread → cbm_pipeline_pass_configlink (v0.5.6, macOS ARM64) #139 / Direct SQLite writer crashes on oversized cells during indexing (repro with netty on macOS arm64) #187 (record-cell side was fixed in fix: implement overflow pages in sqlite_writer to prevent SIGBUS on large records (#139) #175).
• PR fix(extraction): replace fixed traversal stacks with growable arena-allocated stacks #217 (growable AST traversal stacks) addresses a different SIGBUS trigger in the parser.