fix(cli): locate runtime include dir on npm install (#134)

[thiagoralves]

Summary

Fixes #134 — on a global/npm install, --build and --test aborted with Could not locate runtime include directory, forcing users to pass --cxx-flags "-I<pkg>/src/runtime/include" by hand. The headers are shipped; the locator just missed them by one directory level.

Root cause

The package ships the runtime at <pkg>/src/runtime/include (the files entry "src/runtime/"), but the CLI runs from <pkg>/dist/node. findRuntimeIncludeDir only tried:

<dir>/runtime/include            // dist/node/runtime/include   — absent
<dir>/../src/runtime/include     // dist/src/runtime/include     — absent (off by one)

Reaching the shipped headers needs two levels up: ../../src/runtime/include → <pkg>/src/runtime/include. Auto-discovery only worked when run from the project root (where the cwd-relative candidate happens to hit) — never the case on a global install, where cwd is the user's project. The libs locator just below already does the right thing with ../../libs.

Fix

Add the two-level package-relative candidate to both the import.meta (ESM — npm install) and __dirname (CJS bundle / pkg binary) blocks. From dist/node it resolves to <pkg>/src/runtime/include; from the src/node dev layout it resolves to the same src/runtime/include, so one candidate covers both.

Verification

• Reproduced: findRuntimeIncludeDir('') from a foreign cwd returned null before the fix.
• After: it resolves to the shipped src/runtime/include, and strucpp p.st -o p.cpp --build run from /tmp compiles and produces a working binary end-to-end.
• Strengthened the previously-lenient build-utils test (it mocked cwd to /tmp but only asserted "doesn't throw") into a real CLI: --build/--test cannot auto-locate runtime include dir on npm install (off-by-one path) #134 regression that requires the package-relative lookup to resolve src/runtime/include, plus a -I cxx-flags fallback case.
• Full suite green (1999 passed), typecheck + format clean.

🤖 Generated with Claude Code