Bring the snap packaging up to a base that can compile it

[gitosaurus]

The snap packaging had not been touched since January 2021 and could not have
built the current interpreter for two independent reasons.

It was in a place snapcraft does not look. Snapcraft searches exactly four
paths — snapcraft.yaml, snap/snapcraft.yaml, build-aux/snap/snapcraft.yaml,
.snapcraft.yaml. The file was moved from snap/ to drivers/snap/ in e22fc85
(2022-04-07), collateral from an unrelated Boost/GCP restructure, and has been
undiscoverable ever since. Moved back.

Its base predates the language the interpreter is written in. core18 is
Ubuntu 18.04, whose g++ is 7 — nowhere near C++20. core24 brings g++ 13. The
part also still described a make build from before the move to CMake, and
declared version: '2.0' against an interpreter that reports 3.0.

What the rewritten file does:

• plugin: cmake with source-subdir: src, where the CMakeLists.txt lives.
  src/CMakeLists.txt already has an install() rule, so the binary stages
  without an override-build.
• adopt-info reads the version out of src/main.cc at pull time rather than
  repeating it, which is how the old file came to sit five years behind.
• Strict confinement now names its interfaces. A game is a file the player names
  on the command line and a save is written next to it, which is the home plug
  (auto-connected from the store). removable-media is there for games kept on a
  stick and needs snap connect by hand. Classic confinement would have meant
  manual store review for no benefit.
• The three games from the browser build ship pre-compiled, plus the library
  sources — standard.arch and friends — so an author's own game can
  include "standard" by pointing --include at $SNAP/usr/share/archetype/games.
  Compiling a game means running an interpreter, so that part is after: the
  native one, which also makes it the step that cannot be cross-compiled.

The second commit adds a CI job, because snapcraft does not run on macOS and
there was otherwise no way to learn the packaging was broken short of publishing
it. It builds the snap, installs it, and plays a turn of The Gorreven Papers out
of the bundled .acx — the install-and-play half is what checks that the paths
promised in the snap's own description are real. It runs only on changes under
snap/, plus workflow_dispatch; a snap build is minutes in an LXD container,
and a C++ change that will not compile has already failed in CI.

Test plan

Verified locally on macOS as far as macOS allows — snapcraft itself is Linux-only,
so the CI job on this PR is the real test:

• Both YAML files parse.
• The adopt-info extraction returns 3.0 against the real src/main.cc.
• The exact compile loop from override-build run by hand against ./build/archetype:
  gorreven 69,784 bytes, starship 55,703, animal 1,108.
• The CI smoke-test invocation, archetype --perform=…/gorreven.acx, plays a turn.

Not done here

• Publishing. Releasing on tag needs snapcore/action-publish and a
  SNAPCRAFT_STORE_CREDENTIALS secret, and the archetype name registered in the
  store. Worth a follow-up once this builds green.
• --include ergonomics. The interpreter reads no environment variable, so a
  bundled game means typing $SNAP/usr/share/archetype/games. Teaching main.cc
  to honour something like ARCHETYPE_INCLUDE would let the snap set it in
  apps.archetype.environment and make the whole thing a one-word command.

🤖 Generated with Claude Code

https://claude.ai/code/session_019g4fqvhoV5MtLJJXM7uFLZ