Modernization Phase 2.1: modules canary — streamr-eventemitter + streamr-json [iosbuild]

[ptesavol]

The first C++ named modules in the codebase. Both canary packages now carry the exact façade shape the migration is built on (MODERNIZATION.md Part 2):

• One .cppm partition per public header: the header is #included in the partition's global module fragment and its public names are re-exported with export using. GMF entities stay attached to the global module, which is what makes import and #include consumers ODR-safe to mix during the whole migration.
• A primary interface unit (streamr.eventemitter, streamr.json) export imports the partitions — consumers write one import.
• The packages flip INTERFACE → STATIC via streamr_add_module_library() (module interface units are compiled TUs).
• The packages' own tests and the json example now use import — 73 of the 307 tests execute through modules. Everything else in the monorepo keeps #include with zero edits, as designed.

Gate results

Gate	Result
Standalone package builds (BMIs + archive)	✅
export(TARGETS … CXX_MODULES_DIRECTORY …) smoke test (standalone/vcpkg path)	✅ — no fallback needed
Downstream find_package + #include consumer unchanged (streamr-utils standalone)	✅
Root tree full build + tests	✅ 307/307
clangd/lint canary	✅ — better than planned: clangd 22 lints import-using files via the CMake module maps; the planned lint-exclusion fallback was not needed
iOS cross-build of the module libraries	this PR ([iosbuild])

Findings that shape the next phases

1. import doesn't leak transitive std includes the way textual inclusion did — every flipped TU needs to include what it uses (the eventemitter test needed <list>/<tuple>; the example needed <string>). Each package flip will carry a small include-adding pass. A good hygiene side effect.
2. One genuine clangd-modules quirk found: the constrained std::string(std::string_view) constructor template isn't resolved in import-using files (the compiler is fine with it; plain constructors work). Sidestepped in the example with the pass-by-value idiom; documented in MODERNIZATION.md as a watch-item.
3. clangd's secondary diagnostic notes in import-using files can show module-expanded line numbers; primary locations are correct.
4. Lint posture as planned: .cppm gets clang-format only (canary packages' lint.sh extended); headers remain the fully linted source of truth until consolidation.

Next

Phase 2.2: streamr-logger + streamr-utils — folly enters the global module fragment for the first time, and utils is the folly::coro coroutine canary.

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