fix(beam): write package modules to the package's own src/

[dbrattli]

Problem

A NuGet package source in a subdirectory was written to fable_modules/<pkg>/<subdir>/src/ instead of the package's own src/. No OTP app covers that path — {project_app_dirs, [".", "fable_modules/*"]} globs one level, and <subdir>/ has no .app.src of its own — so rebar3 never compiled those files and reported nothing. dotnet fable succeeded, rebar3 compile succeeded, and the first symptom was undef at runtime on a module sitting right there in the output directory.

Project sources were already flat; only package sources mirrored their subdirectory. In one build of Fable.Giraffe's Beam suite, 8 modules were stranded — including all seven Fable.Beam.Otp.* ones, so the bindings library ships a subdirectory of modules unreachable at runtime.

Fix

Write package modules to <pkg>/src/<module>.erl regardless of source subdirectory. The module name already encodes the path (Sinks/Universal.fs → scriptorium_parchment_sinks_universal), so an app is flat by construction and there is no collision risk.

If the output path has no fable_modules/<package> segment to anchor on, this now fails with a FableError instead of falling back to the previous layout — that fallback would silently reinstate the exact bug being fixed, with undef at runtime as the only symptom.

Two alternatives from the report were considered and dropped: populating {modules, [...]} in the generated .app.src validates nothing, since rebar3 fills the module list in the generated .app from the compiled beams and ignores .app.src; and a post-emit assertion would guard an invariant that getOutPath — the only writer — now establishes five lines away.

Verification

In-tree BEAM suite: 2633 passed, 0 failed, plus both entry-point program tests.

Against ../Fable.Giraffe's Beam suite, with its find-based flatten workaround removed:

• zero nested */src/*.erl under fable_modules (was 8 stranded modules)
• all seven fable_beam_otp_* plus scriptorium_parchment_sinks_universal compile into their app's ebin, and code:ensure_loaded/1 resolves them
• suite: 37 passed / 8 skipped
• project layout unchanged: test/shared/RoutingTests.fs → src/shared_routing_tests.erl

No regression test in-tree: reproducing this needs a package with subdirectory sources in fable_modules, which the Beam test projects do not have.

Note

This PR previously also carried an unrelated async-stacktrace fix. That has been split out into its own PR so this one can land on its own evidence.

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