`readonly-close-no-marker` closed the clean-shutdown-marker half of this law and
named the rest as a known residual. This is that residual, closed.
MEASURED on the base: a read-only open → read → close rewrote FOUR files —
`_system/__metadata_field_registry__.json.gz`, `type-statistics.json.gz`,
`subtype-statistics.json.gz` and `verb-subtype-statistics.json.gz`. An IDLE
reader that only opened and closed rewrote all four as well.
The cause was not the closes the marker fix guarded. It was Phase 1 of
closeDurableSteps, where every component flush ran unconditionally. A flush is a
write by definition: MetadataIndexManager#flush() saves the field registry "even
with no dirty fields" (its own comment), and the storage adapter's count flush
re-stamps the three statistics files. A session that committed nothing re-stamped
all four. Phase 2's closes were ungated too — the graph index's close drains both
LSM MemTables to SSTables and stamps its watermark, and the optional
vector/metadata `close` hooks (unimplemented in the reference engine, filled in
by a native provider) persist buffered state.
Every one of those calls now carries the same `!isReadOnly` guard the generation
store already had.
A reader still RELEASES what it holds, so Phase 2 is a branch rather than a skip:
GraphAdjacencyIndex gains `stopBackgroundFlush()`, the non-writing half of its
close, which clears the auto-flush interval that would otherwise outlive the
session. `close()` now calls it too, so there is one place that owns the timer.
Why this matters beyond tidiness: `_system/` is where a store keeps its evidence
about itself — what the writer committed, what the projections have seen. A
reader that rewrites any of it vouches for a state it only observed, and on
shared or snapshot storage it mutates bytes another process owns.
The pin hashes every file under `_system/` (and, in one case, the whole store)
across a reader's open → read → close, names the four paths that used to move so
a regression says which subsystem did it, and asserts the asymmetry holds in the
other direction — a WRITER's close still persists.
