Track bounded simplification of parsing, output, sessions and transaction ownership

[apstndb]

Goal

Make the existing implementation simpler through bounded, independently reviewable PRs. This tracks the remaining September 2026 architecture proposals, concretized against main commit 96a8a60ab5df71447b30db1826d4da2d6677c42b. Each child specifies a chosen implementation, deletion target, behavior boundaries and validation. It is a work sequence, not permission for a broad rewrite.

Already complete

• refactor: consolidate completion caches by kind #908: completion cache map.
• refactor: share DUMP traversal through an explicit writer #909: shared DUMP traversal and explicit internal writer.
• refactor: replace transaction options builder with helpers #910: transaction-options builder removal.

These are completed baselines and must not be implemented again. Existing #380 is reused for source preservation. #790 remains the separate BIGQUERY typed-result contract; #738 and #538 are completed background work, not reopened tasks.

Independently startable work

• Preserve original SQL fragments for SOURCE and batch execution #380 — Preserve original SQL fragments for SOURCE and batch execution
• Pass narrow export options to the shared row writer and remove typed replay settings copies #913 — Pass narrow export options to the shared row writer and remove typed replay settings copies
• Make query-cache publication explicit and independent of SQL table-name detection #914 — Make query-cache publication explicit and independent of SQL table-name detection
• Stop and join progress output before the result sink emits its first bytes #916 — Stop and join progress output before the result sink emits its first bytes
• Represent each Result body with one closed variant while retaining shared metadata #917 — Represent each Result body with one closed variant while retaining shared metadata
• Give each Session a fixed connection identity independent of live registry settings #919 — Give each Session a fixed connection identity independent of live registry settings
• Replace the single-use TransitTransaction wrapper with a direct pending transition #921 — Replace the single-use TransitTransaction wrapper with a direct pending transition
• Verify delayed heartbeat ticks cannot act on a replacement transaction owner #922 — Verify delayed heartbeat ticks cannot act on a replacement transaction owner

Work with prerequisites

• Share native statement-kind mapping between semantic and lexical parsing #912 — Share native statement-kind mapping between semantic and lexical parsing
• Replace per-query systemVariables copies with narrow rendering overrides #915 — Replace per-query systemVariables copies with narrow rendering overrides
• Pass operation output explicitly through statement execution and remove Session output mutation #918 — Pass operation output explicitly through statement execution and remove Session output mutation
• Validate replacement sessions before publishing live connection values and callbacks #920 — Validate replacement sessions before publishing live connection values and callbacks
• Preserve the logical transaction context across pending activation and own SET LOCAL undo there #923 — Preserve the logical transaction context across pending activation and own SET LOCAL undo there
• Store automatic DML on its transaction context and remove parallel generation bookkeeping #924 — Store automatic DML on its transaction context and remove parallel generation bookkeeping

Sequence and implementation boundaries

• Export-writer inputs + explicit query-cache publication → query rendering overrides → explicit operation output. Progress stop/join also precedes explicit operation output.
• Fixed session construction identity → candidate validation and adoption.
• Direct pending transition + completed heartbeat investigation → stable logical transaction lifetime → automatic-DML queue ownership.
• Source-envelope changes land before parser-mapping cleanup. Result-body migration lands before the broad explicit-output signature migration. Both orders are recorded as blocked-by links so implementations do not overlap in the shared files; these are sequencing prerequisites, not semantic contract dependencies.
• Use separate worktrees per PR. Parallelize independent preparation; serialize heavy gates and integrate changes against refreshed main. Keep each issue's old path removal in the same PR as its replacement.

The heartbeat item is an unreproduced scheduling hypothesis with a deterministic investigation plan. It is not a confirmed bug or a reason to rewrite locking. The query-cache item identifies a source-backed behavior discrepancy and requires a red/green regression before its fix. Other items are structural refactors except the explicitly documented SOURCE/batch echo and pre-output progress improvements.

Completion criteria

• Every child has a merged PR satisfying its own behavior and deletion criteria, or a documented evidence-based disposition for the investigation.
• No temporary parallel representations, obsolete settings copies or ownership shims remain from this series.
• Existing output, transaction, SET LOCAL, feature, DUMP and MCP contracts remain covered; intentional behavior changes are isolated and explained.
• Required checks and independent current-head review are recorded per PR. No performance or correctness claims are inferred from aggregate architecture review.

No package extraction campaign, single-implementation interfaces, provider-neutral result framework, dependency upgrades, release or external-repository issue creation is included.