fix: guard circular import/load_file against C-stack SIGSEGV (#496)

[InauguralPhysicist]

Problem

A mutual or self-referential import (a→b→a) or load_file (a↔b) recursed through vm_execute until the C stack overflowed — SIGSEGV, rc=139, uncatchable. The module cache is only populated after a load finishes, so a re-entrant load of a still-loading path missed the cache and recursed.

Reproduced on v0.28.0:

# a.eigs: import b   |   b.eigs: import a   |   main.eigs: import a
$ eigenscript main.eigs
Segmentation fault (core dumped)   # rc=139

Fix

A per-EigsState in-flight load stack (eigs_loading_active / enter / leave) records paths whose load is currently on the C stack:

• the import opcode checks it on cache-miss;
• load_file (which has no cache and re-executes every call) keys on the canonical realpath;
• shared by both, so a cycle that crosses the two (import → load_file → import) is caught.

On a cycle the loader raises a catchable io error instead of recursing:

import: circular dependency — 'a' is already being loaded

Repeated sequential loads of the same file stay legal — the entry pops when each load completes; only active re-entrancy is a cycle. Diamond imports (a→c, b→c) still resolve c once via the cache.

Tests

New suite section [115] (generated multi-file fixtures): mutual import raises + is try/catch-able, mutual load_file raises, none segfault.

• Release suite: 2625/2625
• ASan + detect_leaks=1: 2625/2625, leak tally 0

Closes #496

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