Export pgoutput.so imports from the main wasm module to enable logical replication slots

[riderx]

Problem

Logical replication slots don't work in PGlite (electric-sql/pglite#880). Even with wal_level=logical, consuming a slot crashes the session:

SELECT pg_create_logical_replication_slot('slot1', 'pgoutput'); -- works
SELECT * FROM pg_logical_slot_peek_binary_changes('slot1', NULL, NULL,
  'proto_version', '1', 'publication_names', 'pub1');
-- returns 0 rows, and every query afterwards silently returns nothing,
-- until the backend PANICs with "ERRORDATA_STACK_SIZE exceeded"

Root cause

pgoutput.so is loaded at runtime as an emscripten side module, but unlike pgxs-built extensions (which generate a .imports file at install time that feeds -sEXPORTED_FUNCTIONS via the pglite-exported-functions target), it is linked from src/backend/replication/pgoutput via Makefile.shlib, so its imports were never collected.

With -sMAIN_MODULE=2, every symbol used only by pgoutput gets dead-code-eliminated from the main module — 46 functions in the current build, including defGetStreamingMode, all logicalrep_write_*, GetRelationPublications, CacheRegisterRelSyncCallback, etc. (verified by diffing WebAssembly.Module.imports(pgoutput.so) against WebAssembly.Module.exports(pglite.wasm); GOT.mem resolves fully, only function imports are missing).

At runtime the first missing call happens inside pgoutput's startup callback, throws TypeError: r is not a function from an invoke_* trampoline, and the JS side swallows it, leaving the backend permanently corrupted.

Fix

Generate pgoutput.imports at install time, using the same llvm-nm recipe as pgxs.mk, so the existing pglite-exported-functions target picks it up and the symbols survive DCE.

After rebuilding, all 46 previously-missing imports are exported from the main module and logical decoding with pgoutput works end-to-end (create/list/drop slots, peek/get binary changes, slot persistence across dump/load). Tests exercising this live in the companion PR to electric-sql/pglite, together with defaulting wal_level=logical and no longer swallowing unexpected WASM exceptions.

Fixes electric-sql/pglite#880

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