Fix debug and release-small build variants (#36)

[lukashes]

Problem

zig build debug and zig build release-small fail to compile: both build main.zig with no module imports and no rdkafka link, so main's imports are unresolved. Only the default zig build was wired correctly.

Solution

• Add a wireMainExe helper that applies main.zig's module imports and C links (pq, rdkafka, libc) to an executable's root module.
• Use it for the release, debug, and release-small variants so they can't drift apart again.

Split out of #70, which now covers only the dead-code removal.

Verified: zig build, zig build debug, zig build release-small all produce binaries (outboxx, outboxx-debug, outboxx-small).

Closes #36

[github-actions]

📊 Benchmark Results

Current run is the minimum over 3 passes, compared against the base branch (main @ 39e25a5), built on the same runner.

Benchmark	Baseline	Current	Δ Time	Allocs	Status
Converter DELETE	134.55μs	134.83μs	+0.2%	12 → 12	➡️
Converter INSERT	130.41μs	130.54μs	+0.1%	12 → 12	➡️
Converter UPDATE	182.23μs	183.06μs	+0.4%	21 → 21	➡️
JsonSerializer	44.55μs	44.95μs	+0.9%	3 → 3	➡️
KafkaProducer produce	327.94μs	332.46μs	+1.4%	1 → 1	➡️
KafkaProducer sendMessage	390.82μs	399.79μs	+2.3%	0 → 0	➡️
PgOutputDecoder	101.97μs	101.69μs	-0.3%	6 → 6	➡️
getPartitionKeyValue boolean	16.90μs	16.86μs	-0.2%	1 → 1	➡️
getPartitionKeyValue integer	53.03μs	53.08μs	+0.1%	3 → 3	➡️
getPartitionKeyValue not found	0.03μs	0.03μs	+0.0%	0 → 0	⚪ noise
getPartitionKeyValue string	18.99μs	18.99μs	+0.0%	1 → 1	➡️
matchStreams found	19.21μs	19.23μs	+0.1%	1 → 1	➡️
matchStreams not found	0.17μs	0.05μs	-69.0%	0 → 0	⚪ noise

Summary: ➡️ 11 neutral · ⚪ 2 ignored (sub-μs)

_{Thresholds: <1μs ignore · 1–20μs 15% · 20–50μs 10% · ≥50μs 5%. Measured on a shared CI runner — treat small deltas as noise. Informational only; this check never fails the build.}