db: invalidate copied pooled connection handles on release

[medvednikov]

Copying a pooled db.Conn copied its driver pointer while sharing its mutex. Releasing one copy left other copies able to use or return the physical connection after another borrower acquired it. Store the driver and mutex in shared private state so every copy becomes invalid together, and concurrent release/discard returns or closes the driver exactly once.

Add stale-copy and concurrent-copy regressions and document the release behavior. This addresses a safety gap in the existing pooling API discussed in #27668; the remaining multi-phase proposal is outside this change.

Validation:

• Reproduced the stale-copy query against SQLite on unchanged master; the fixed copy returns db: connection is released.
• Pool tests passed.
• ./v -cstrict -cc clang -silent test vlib/db/pool_test.v vlib/db/tx_test.v vlib/db/driver_test.v passed (3/3).
• Formatting, ./v check-md vlib/db/README.md, and git diff --check passed.

Refs #27668.

Focused validation at c4ef4adcc264 after the upstream merges: all five DB files remain byte-identical to fix 98eac560e538. Rebuilt the compiler after the first upstream compiler merge and passed strict Clang pool, transaction, and driver tests (3/3). The subsequent merge changed only doctor code/documentation and its fixture; the same three tests passed again with the compiler held stable. These checks use test drivers; live external database services were not exercised.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-09T03:28:46.644846Z	c4ef4ad	New commits

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