perf(cypher): stream query execution chunk by chunk from a paged node scan

On the kernel index (8.5 M nodes, 1.9 M CALLS edges) `MATCH (a)-[:CALLS]->(b)
RETURN count(*)` peaked at 18-25 GB RSS (killed in a 12 GB container): the
store materialised every start node with all its strings (4.9 GB), every one
was deep-copied into a 1.6 KB binding, and every expanded row was materialised
again before projection.

cypher: execute_single now runs the start nodes through the pipeline in chunks
of 1024 (pattern-0 hops, late WHERE, WITH, RETURN). Each hop emits a binding's
expansions in input order and every consumer (RETURN rows / star / default
projection / aggregation, WITH aggregation / projection) keeps its state
between chunks, so the concatenated output is the single-pass output: same
rows, row order, first-seen group order, ORDER BY ties, DISTINCT, LIMIT and
max_rows truncation, OPTIONAL fallback rows, warnings. Queries with additional
MATCH patterns keep one pass (their join strategy depends on the first binding
of the whole set). Start nodes and fetched neighbours move their strings into
the binding instead of being deep-copied and freed.

store: cbm_store_node_scan_open/_next/_close hand out the rows of
cbm_store_find_nodes / _by_label a page at a time. Pages are keyset queries
along the index the eager query walks (idx_nodes_file: NULL file_path rows by
id, then the rest of the last file by id, then the files after it;
idx_nodes_label: by id), each reset before it returns, so no read lock is held
between pages. open verifies the eager plan with EXPLAIN QUERY PLAN and falls
back to the eager rows for any other plan.

Kernel, same index, medians of 3 (macOS arm64):
  count(*)    peak RSS 24.9 GB -> 256 MB,  23.0 s -> 13.0 s
  top-callee  peak RSS 12.4 GB -> 1.26 GB, 11.7 s -> 10.9 s
Responses byte-identical to the baseline on django, redis and the kernel
(19 Cypher + 24 tool cases each); the streamed scan equals the eager scan
field by field, in order, for every project and label of all three indexes.

A scan that fails part-way fails the whole query ("query aborted: reading the graph failed") instead of answering from the pages read so far:
The streamed start-node scan answered from the rows of the pages it had read
when a later page failed (a SQLite step error or a page re-prepare failure):
a partial result that looked complete. A failed page, a scan that cannot
open, or a failed eager fallback now marks the start source failed;
execute_chunk returns an error and cbm_cypher_execute reports
"query aborted: reading the graph failed (<store error>)" with no rows. The
store error is logged as cypher.scan_failed.

The test seam cbm_store_node_scan_test_fail_page_after (CBM_ENABLE_TEST_SEAMS)
fails one page after a given number of successful pages; the new test fails
the second page of a label scan (rows and count(*)) and a later page of the
all-nodes scan, and asserts an error result with no rows.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

Author: DeusData

scripts/memory-core-baseline.txt

@@ -24,7 +24,7 @@ src/cli/config_text_edit.c	72
 src/cli/config_toml_edit.c	73
 src/cli/config_yaml_edit.c	117
 src/cli/hook_augment.c	31
-src/cypher/cypher.c	128
+src/cypher/cypher.c	125
 src/daemon/application.c	107
 src/daemon/bootstrap.c	13
 src/daemon/daemon.c	31