docs: move root .md files into docs/ + update every reference

Picks up the file renames that landed alongside the previous commit
and fixes everything that pointed at the old root locations:

- README.md doc-map updated: every entry now points at docs/X.md,
  with a new "docs/" lead entry pointing at the guided-tour index.
- README inline cross-refs (ARCHITECTURE / INTEGRATION / SECURITY /
  BENCHMARKS / MES_INTEROP / PROOFS) repointed to docs/.
- README "Interop" section rewritten — used to mention only
  secsgem-py; now covers all four external validators (secsgem-py
  31 / secs4java8 55 / tshark 69 frames / libFuzzer 200 k+ runs)
  with a one-line summary each, plus pointers to interop/README.md
  and docs/VERIFICATION.md.
- README "Deferred follow-ups" cleaned: dropped the explanatory
  "Listed here so reviewers don't go looking for them in
  COMPLIANCE.md and find an 'out of scope' entry that sounds
  defensive" sentence — the section header speaks for itself.
- docs/00_index.md "Where the rest of the docs live" table: dropped
  every `../` prefix since the docs are now siblings.
- docs/01_what_is_secs_gem.md PROOFS reference updated to sibling.
- docs/02_the_cast.md INTEGRATION + MES_INTEROP refs updated to
  siblings; dropped the stale "at the repo root" wording.
- interop/README.md: VERIFICATION + PROOFS refs updated to
  ../docs/X.md; stale "~24 + 4 checks" updated to 31 (matches
  PROOFS.md and README).
- examples/pvd_tool/README.md: every doc cross-ref now points at
  ../../docs/X.md.
- Source / data / CI comments mentioning doc names (e.g.
  "INTEGRATION.md §3", "COMPLIANCE.md gap") rewritten to
  "docs/INTEGRATION.md §3" etc. — affects 9 files across
  include/, apps/, tests/, data/, examples/, .gitea/workflows/.

Verified: full build under docker passes, 445/445 test cases pass,
2 753/2 753 assertions pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_data_model.cpp

@@ -35,7 +35,7 @@ TEST_CASE("ECID set rejects unknown id and out-of-range values") {
   CHECK(ecids.get(10)->value == s2::Item::u4(uint32_t{50}));
   CHECK(ecids.set_value(999, s2::Item::u4(uint32_t{1})) == EquipmentAck::Denied_UnknownEcid);
 
-  // Out-of-range rejected (closes COMPLIANCE.md gap).
+  // Out-of-range rejected (closes docs/COMPLIANCE.md gap).
   CHECK(ecids.set_value(10, s2::Item::u4(uint32_t{5000})) == EquipmentAck::Denied_OutOfRange);
   CHECK(ecids.get(10)->value == s2::Item::u4(uint32_t{50}));  // unchanged
 }

tests/test_hsms_gs_integration.cpp

@@ -4,8 +4,8 @@
 // its own router + selected handler + send path.
 //
 // This is the "two MES, one tool" pattern that the codebase advertises
-// in COMPLIANCE.md §1 ("HSMS-GS (general-session) ✅") and that
-// INTEGRATION.md §7 mentions.  Until now there was no end-to-end test
+// in docs/COMPLIANCE.md §1 ("HSMS-GS (general-session) ✅") and that
+// docs/INTEGRATION.md §7 mentions.  Until now there was no end-to-end test
 // covering the Server/Client integration — only direct Connection
 // wire tests in test_hsms_gs.cpp.
 

tests/test_thread_safety.cpp

@@ -2,7 +2,7 @@
 //
 // EquipmentDataModel is single-threaded by design — there are zero
 // locks anywhere in the store hierarchy.  The contract documented in
-// INTEGRATION.md §3 is: all access (reads from the dispatcher, writes
+// docs/INTEGRATION.md §3 is: all access (reads from the dispatcher, writes
 // from the application) must run on the io_context that drives the
 // HSMS connection.  Cross-thread updates marshal through `asio::post`.
 //