[thekevinbot]

Fixes #1131. Part of #1072.

Both bindings carried the same arm-for-arm match over RowEvent's four
variants, producing the same {table, action, row, old_row, error, file_path}
record. That decision is language-neutral, so it moves into the core as
dirsql::flatten_row_event (#[doc(hidden)], packages/rust/src/row_event_flat.rs),
the shape extension_resolution established. Each binding keeps only its
marshaling half: value_row_to_py_dict over the borrowed rows plus the GIL
step for PyO3, value_row_to_js plus the unsafe-integer error for napi.

Rows are borrowed (Option<&'a Row>), so no clone is added in the core; the
clone stays at the binding boundary where it was already paid.

PlainRowEvent / row_event_to_plain are gone from packages/python/src/lib.rs,
and the four Python tests that only restated the flattening moved into the core
(joined by a fifth: Error with a named table, which neither binding covered).
The napi tests stay put — they assert on the napi RowEvent struct and cover
value marshaling and the unsafe-integer error path, which is the binding's own
half, not the flattening.

No public API changed in any SDK. No migration fragment.

One shape note

row_event_flat.rs imports Value at module level under #[cfg(test)] rather
than inside the test module: unit lint's no-out-of-module-import rule
forbids a unit test importing crate::db::Value directly, and an unconditional
module-level import would be unused in a non-test build. This mirrors how
differ.rs tests reach Value through use super::*.

Cross-SDK Parity

Parity-restoring, not drift: no SDK's observable surface changes, and the
flattening that was implemented twice is now implemented once for both.
PARITY.md gains a "Where the row-event flattening lives" note under Core
Types, matching the extension_resolution note. No follow-up issues needed.

Manual verification

Built both bindings and watched a real directory through each SDK, observing
all four variants.

Python (uv run maturin develop --uv, then a script driving db.watch() over
a temp dir):

insert: table='items' row={'id': 1, 'name': 'one'} old_row=None error=None file_path='a.json'
update: table='items' row={'id': 1, 'name': 'two'} old_row={'id': 1, 'name': 'one'} error=None file_path='a.json'
delete: table='items' row={'id': 1, 'name': 'two'} old_row=None error=None file_path='a.json'
error: table='items' row=None old_row=None error='JSONDecodeError: Expecting value: line 1 column 1 (char 0)' file_path='b.json'

Node (pnpm build, then a script driving for await (const e of db.watch())):

insert: table="items" row={"name":"one","id":1} oldRow=undefined error=undefined filePath="a.json"
update: table="items" row={"name":"two","id":1} oldRow={"id":1,"name":"one"} error=undefined filePath="a.json"
delete: table="items" row={"id":1,"name":"two"} oldRow=undefined error=undefined filePath="a.json"
error: table="items" row=undefined oldRow=undefined error="GenericFailure, Unexpected token 'o', \"not json\" is not valid JSON" filePath="b.json"

Same field values, same action strings, same error path as before the move.

just preflight (with nightly on PATH for the Rust coverage gate): 0 failing
pairs, 3 skipped (packaging). The Rust coverage floors did not move — the
change adds fully-covered core lines and removes none, so no re-pin of
[rust.coverage].

E2E Verification

• Ran e2e suites locally for every affected SDK
• Python SDK e2e: pass (37 passed)
• TypeScript SDK e2e: pass (17 passed, 5 files)
• Rust core e2e (if applicable): pass (cargo test --workspace)
• packages/python/e2e-attestations/<branch>.json receipt written if packages/python changed (just e2e-attest-python)
• packages/ts/e2e-attestations/<branch>.json receipt written if packages/ts changed (just e2e-attest-ts)
• Command(s) run: just e2e-attest-python, just e2e-attest-ts, cargo test --workspace, just preflight
• Result summary: all green; both receipts committed.

Changelog / Migrations

• Changelog fragment added under <root>/<pkg>/changelog.d/ for each changed package (packages/rust, packages/python, packages/ts)
• Migration fragment added under <root>/<pkg>/migrations.d/ (or: not required -- additive/bugfix only) — not required: no public API change, no behavior change