Overhaul import loading semantics

[rTreutlein]

Summary

Overhaul file imports by combining the strongest parts of the existing import PRs into one coherent loader:

• propagate errors from imported MeTTa and Python files;
• recursively pre-register imported function signatures so import order does not affect compilation;
• resolve nested relative imports from the importing file;
• canonicalize import paths;
• make imports idempotent per target space;
• terminate cyclic imports using explicit loading and loaded states;
• clear failed import state so a repaired source can be retried;
• raise an explicit existence_error(source_sink, ...) for missing files;
• move git-import! from lib_import into the core loader;
• prepare core Git dependencies before signature discovery, then safely no-op when their normal runnable is encountered;
• resolve library names to the first library root containing the requested source.

The signature prescan remains separate from normal module execution, so imported atoms, clauses, and top-level runnables are still processed normally. git-import! is the one core dependency primitive prepared before that scan because it can make later imports resolvable.

This consolidates ideas explored in #157, #159, #161, #171, #189, and #195. It addresses #154, #180, #181, and #196.

Validation

bash test.sh
# passed, including a cold clone in examples/git_import.metta
# and the network-free examples/git_import_core.metta regression

cd python
uv run pytest tests
# 4 passed

[rTreutlein]

Signature prescanning is causing more trouble than it's worth so i will be removing it and creating a new PR.