[omdxp]

Summary

• Wires -memcheck/-memcheck-leaks into fun test [<path>]. Leak reporting is off by default under -memcheck (opt in with -memcheck-leaks): a short-lived process - every CLI invocation this compiler makes, and every -test binary - never has to free a transient allocation before it exits a moment later anyway, so treating that as a leak flags ordinary code, not a bug. Double-free/use-after-free/buffer-overflow stay caught unconditionally either way.
• Three real bugs found and fixed while getting the whole suite to actually pass under -memcheck:
  • A hoisted temp's own #line directive (under -g) could read an uninitialized Pos when the enclosing statement's own directive had already been flushed, producing a real null-pointer crash. Caught because -memcheck implies -g for real file:line reports.
  • Compiling one of std.memcheck's own dependencies directly under -memcheck (every stdlib file, in this project's own test corpus) reached that exact file a second time through the use std.memcheck the feature injects, double-merging its declarations.
  • std.memcheck itself calling into std.io/std.string created a circular forward reference when the entry file was the one supplying those declarations - fixed by building its own reports with std.c.io's snprintf/fprintf instead, dropping the dependency entirely.
  • Bonus, unrelated: an fls debug-log line passed a 64-bit length where printf's %.*s precision expects a 32-bit int - undefined behavior, just never hit until this branch's own heavier compiler runs exercised it.
• CI: a new -memcheck step on the ubuntu lane, manual-trigger only (workflow_dispatch, the GitHub Actions equivalent of GitLab's manual job) - the guard-page allocator's own per-allocation mmap/mprotect cost is too slow to add to every push/PR, but it's there on demand.
• Docs: a "Memory Checker" section in docs/tooling.md, and a "Ternary Conditional" section in docs/language.md (catching up docs for the ternary feature from the previous PR).

Test plan

• fun test . passes 56/56, run twice, after a fresh 3-stage bootstrap from the installed release
• -fmt-check-all clean, fun lint src/fun lint stdlib clean
• scripts/run_examples.sh: 222 total, 0 failed, 21 expected-fail, 0 unexpected-pass (matches baseline)
• VS Code extension smoke test: 10/10 passing
• Every stdlib file, codegen_tests.fn's 125 tests, and the bulk of the suite verified passing under -memcheck directly (the full suite run is slow enough - guard pages are much slower than a real malloc - that a single local run reliably completing end to end on this machine wasn't practical; the new manual-trigger CI step is the real full-suite gate going forward)
• New regression tests: a hoisted temp's #line directive under -g, -memcheck/-memcheck-leaks flag parsing, leak-vs-double-free behavior under -test, and compiling a std.memcheck dependency directly under -memcheck