Let the environment say where the library lives

[gitosaurus]

ARCHETYPE_INCLUDE is a colon-separated search path, in the shape PATH taught
everyone to expect. It is consulted after --include, so an explicit flag
still wins, and after the source file's own directory, which primarySource has
always pushed to the front — so nothing changes for a game sitting beside the
library it includes.

The motivating case is a packaged interpreter. The snap in #35 knows perfectly
well where standard.arch ended up inside its own squashfs; without this the
player has to know too, and type
--include=/snap/archetype/current/usr/share/archetype/games. A package can set
the variable once and let archetype --source=mygame.arch simply work.

Two edge cases, both deliberate:

• Set but empty counts as unset, the way an empty environment variable
  usually does.
• An empty entry within a list still means the current directory, the way it
  does in PATH. So --include=a: is unchanged.

The split is done on the string_view in place rather than through an
istringstream, so the only strings allocated are the ones actually kept — which
addSearchPath takes by value and moves.

The debug trace

SHOW(path) is gone, and the answer to "wasn't that debug-only?" is yes, and
that was not enough. It was guarded by #if NDEBUG, but cmake -S src -B build
— the build this project documents — sets no CMAKE_BUILD_TYPE, so CMake passes
no -DNDEBUG and the guard never fired. It has printed path == games on every
compile anyone has run. With that call removed the macro had no users left in
main.cc, so it went too. (TestExpression.cc defines its own, untouched.)

Test plan

• ./build/archetype --test — 17 suites, 0 failures.
• Out-of-tree source (mygame.arch in a temp dir, library in games/), which is
  the case the snap cares about:
  • no include at all → fails, Cannot open source file "standard"
  • ARCHETYPE_INCLUDE only → compiles
  • ARCHETYPE_INCLUDE="" → fails, i.e. treated as unset
  • ARCHETYPE_INCLUDE=/nonexistent:games → compiles, second entry found
  • --include present with a bogus env value → compiles, flag wins
• The .acx produced via the variable is byte-identical to the one produced
  via the flag, and to the in-tree compile: 69,784 bytes.

Once this lands, #35 should gain an environment: stanza setting the variable,
and lose the long path from its description.

🤖 Generated with Claude Code

https://claude.ai/code/session_019g4fqvhoV5MtLJJXM7uFLZ