fix(scanner): repair SD022 compile error breaking estate-wide Hypatia scan

[hyperpolymath]

Problem — main does not compile, so every repo's scan is red

The Hypatia reusable workflow git clones this repo and runs mix escript.build before scanning. As of #545 (7df53b1b), main fails to build:

== Compilation error in file lib/rules/structural_drift.ex ==
** (CompileError) undefined function real_subdirs/0

So scan / Hypatia Neurosymbolic Analysis dies at "Build Hypatia scanner" on every consumer repo in the estate (surfaced while verifying #464 — the cache pin is fine, this is the real blocker).

Root cause

#545 rewrote sd022_stale_path_after_rename/1 and renamed the binding real_subdirs → real_basenames (now produced by src_dir_index/1), but left a dangling real_subdirs reference in the final Enum.reject. Two further latent defects from the same half-finished refactor:

1. The regex uses a negative lookbehind with one capture group, but the pipeline destructured [_, prefix, dir] (two groups) and called real_path_reference?/4 — a FunctionClauseError waiting to fire once the compile error was cleared.
2. src_dir_index/1 switched to git ls-files, which cannot see an empty src/<dir> directory — so the early-return guard would wrongly short-circuit (this is exactly what the existing test "flags docs referencing src/<dir>/…" sets up).

Fix (minimal, behaviour-preserving)

• real_subdirs → real_basenames in the final reject.
• Collapse the vestigial 2-group prefix path to [_, dir] and remove the now-unused real_path_reference?/4 + foreign_prefix?/2 (the lookbehind already excludes prefixed/foreign refs, so they were dead).
• Guard now also checks File.dir?(repo/src) so an empty src/<dir> still counts.

Net diff: +4 / −25 in one file.

Verification

Ran the real suite locally (Elixir 1.14 / OTP 25):

mix test test/structural_drift_test.exs
32 tests, 0 failures

(All four sd022_* tests pass, plus elixirc compiles the module clean — reproduced the original CompileError first, confirmed it's gone after.)

Once main compiles again, the estate-wide scan lane unblocks. Closes the scan-side of #464.

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Drafted by Claude Code; please run mix test / mix escript.build in CI before merge.

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Generated by Claude Code