Source: extracted verbatim from the Engineering PRD, docs/prd/03-build-system.md (REQ-DOC-004). From M0 onward this file is the canonical, living ADR record; status changes happen here and in the index, with the PRD text remaining the historical record.
Status: Accepted.
Context: Charter T4 demands three consumption modes including a two-file drop-in; Charter §7.3 demands -fno-exceptions compatibility; ADR-003 requires per-ISA compiled code.
Problem: header-only vs compiled vs compiled+amalgamation.
Alternatives:
Header-only. + zero build integration. − every consumer TU recompiles all ISA paths; target-region pragmas inside consumer-included headers leak diagnostics and break under MSVC; dispatch table initialization becomes ODR-fragile; compile-time cost contradicts vendorability in practice.
Compiled static library only. + clean. − no two-file vendoring path; fails Charter §6.5.
Compiled library + generated amalgamation (selected). + normal builds stay clean; vendoring path is the proven simdjson/fsst pattern (OA §3, §8). − amalgamation generator must be maintained and verified (mitigated by REQ-BUILD-013's identical-output gate).
Decision: Alternative 3.
Consequences: MOD-AMALG exists (M8); release assets carry the pair (REQ-REPO-011).
Reconsideration: if C++ modules become the ecosystem vendoring norm.
Related: REQ-BUILD-002/010/013, ADR-003, ADR-018, Charter T4/§6.5.