Phase 0. Spec freeze and skeleton trees

Field	Value
MEP	MEP-45 §Phases · Phase 0
Status	IN PROGRESS
Started	2026-05-22 (GMT+7)
Landed	—
Tracking issue	#22066
Tracking PR	#22067

Gate

This MEP merged on main; transpiler3/c/{aotir,lower,emit,build,toolchain/zig,runtime/{include,src}}/doc.go compile clean and report zero tests; tests/transpiler3/c/ exists with a README.md; implementation tracking pages for every phase exist under /docs/implementation/0045/; sidebar entries visible on the website.

Goal-alignment audit

The user-facing goal of MEP-45 is "ship a Mochi program as a single native binary on every tier-1 triple". Phase 0 does not move that goal directly; it is paperwork that costs one PR and unlocks every later phase. Specifically, after Phase 0 a contributor can answer four questions without reading the MEP end-to-end: (1) which Go package owns each pipeline stage (the doc.go tree), (2) where fixtures live and how they are named (tests/transpiler3/c/README.md), (3) which phase is responsible for which language surface (the §Phases table in MEP-45), and (4) where the per-phase rolling status lives (the tracking pages under /docs/implementation/0045/). Without those four anchors, every later phase repeats the same orientation cost. Aligns.

Sub-phases

#	Scope	Status	Commit	PR
0.0	MEP-45 merged with refactored framing, §Phases section, implementation tracking docs, sidebar wiring	IN PROGRESS	—	#22067
0.1	transpiler3/c/{aotir,lower,emit,build,toolchain/zig,runtime/{include,src}}/doc.go compile clean	IN PROGRESS	—	#22067
0.2	tests/transpiler3/c/README.md documents fixture layout and naming convention	IN PROGRESS	—	#22067

Decisions made

• Eight doc.go files, not one. Each subpackage gets its own godoc-style narrative so that go doc mochi/transpiler3/c/lower is a useful first page for a contributor. The root transpiler3/c/doc.go carries only the pipeline diagram and cross-references.
• runtime/include/doc.go and runtime/src/doc.go exist even though those dirs hold only C. Keeping them as Go packages means go vet ./transpiler3/c/... walks the subtree without erroring and go doc surfaces the runtime layout next to the Go packages.
• Fixture directory layout is phase-grouped. tests/transpiler3/c/fixtures/<phase-area>/<fixture-name>/ puts hello next to other Phase 1 fixtures, match next to other Phase 4 fixtures, and so on. The phase ownership is visible in the path; no extra registry file required.
• Per-target expected output uses expect.<triple>.txt overrides. Phase 11 needs cross-target output flexibility for rare cases (byte order, native int width). The harness picks the most-specific expect.*.txt match; default expect.txt covers everything else.

Deferred work

• Per-phase substrate vendoring (BDWGC, mimalloc, minicoro, cwisstable, yyjson, libcurl, utf8proc / simdutf) is documented in runtime/doc.go but not yet imported. Each phase pulls in the substrate piece it needs (Phase 3 for cwisstable, Phase 9 for minicoro, Phase 14 for libcurl, ...).
• transpiler3/c/toolchain/zig/manifest.go SHA-256 pins are deferred to Phase 1.3, when the download path lands.

Closeout notes

Fill in after gate goes green.