L-7b-ii follow-up: add Linux CI build job for libtotapi_mono.so

[k-yoshimi]

Context

PR #202 landed libtotapi_mono.so as a macOS-verified PoC. The Makefile branches on platform via ifeq ($(UNAME_S),Darwin) for SO_SONAME_MONO, and reuses SO_GROUP_BEGIN/END + SO_UNDEFINED from the existing libtotapi.so block which already has different Darwin / Linux flag sets:

ifeq ($(UNAME_S),Darwin)
  SO_GROUP_BEGIN =
  SO_GROUP_END =
  SO_UNDEFINED = -Wl,-undefined,dynamic_lookup
else
  SO_GROUP_BEGIN = -Wl,--start-group
  SO_GROUP_END = -Wl,--end-group
  SO_UNDEFINED = -Wl,--unresolved-symbols=ignore-all
endif

The mono target reuses these (no duplication), but the Linux branch is untested for the mono path — the existing python-tests.yml workflow only builds the default libtotapi.so. Linux ld differs from Apple ld in archive-ordering semantics and unresolved-symbol handling; a flag combination that works on macOS may fail on Linux with link errors, dlopen errors, or silent corruption.

Scope

Add a build-only CI job (or extend an existing one) that runs on Ubuntu:

make -C tot libtotapi_mono.so
make -C tot libtotapi_mono_inspect

Acceptance:

• libtotapi_mono.so builds on Linux x86_64 with gfortran 13.x.
• The 8-bpsd-storage-symbol invariant in libtotapi_mono_inspect passes.
• ldd libtotapi_mono.so shows no lib*api.so deps (only system libs + libgfortran).

This is build-only validation. The 1e-10 equivalence and BPSD smoke tests against the mono build are deferred to #201 (which adds the unified graphics stubs needed to make it dlopen-loadable).

Why now vs later

Filing this now (vs as part of #201's Phase 2a) so:

1. Linux linker incompatibilities surface BEFORE L-7b-ii Phase 2: monolithic mode + BPSD coupling activation (umbrella) #201 work depends on them — fixing them early avoids rework in Phase 2a's PR.
2. The mono target invariants (8 bpsd symbols, no .so deps) get periodic CI regression checking.

Effort

~30 min: copy the existing macOS-tested job's setup steps, add the two make invocations, gate on PR-changing-the-mono-target paths.

References

• PR chore(tot): L-7b-ii monolithic libtotapi_mono.so PoC #202 (PoC landing)
• tot/Makefile lines 575-665 (mono target + inspect helper)
• Findings doc: docs/superpowers/specs/2026-05-14-l7b-ii-monolithic-poc-findings.md
• Codex retrospective review 2026-05-15 (this is item 4)