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>

interop/README.md

@@ -5,14 +5,15 @@ take our word for it.  Each one validates our C++ codec / framing /
 dispatch against an **independent third-party implementation** that
 read the SEMI standards without talking to us.
 
-See [`../VERIFICATION.md`](../VERIFICATION.md) for the full test plan
-and the honest accounting of which proofs are external vs internal.
+See [`../docs/VERIFICATION.md`](../docs/VERIFICATION.md) for the full
+test plan and the honest accounting of which proofs are external vs
+internal.
 
 ## What's here
 
 | Validator                                  | Independence                                | Coverage                                                                                                            |
 |--------------------------------------------|---------------------------------------------|---------------------------------------------------------------------------------------------------------------------|
-| `host_vs_cpp_server.py` + `passive_equipment.py` | secsgem-py 0.3.0 — Python reference impl   | ~24 + 4 checks: S1, S2, S5, S6, S7, S10 happy paths                                                                  |
+| `host_vs_cpp_server.py` + `passive_equipment.py` | secsgem-py 0.3.0 — Python reference impl   | 31 checks: S1, S2, S5, S6, S7, S10 happy paths + unsolicited S6F11 / S5F1                                          |
 | `raw_gem300_harness.py`                    | secsgem-py with hand-crafted SecsStreamFunctions | 3 checks: S3F17, S16F5, S16F27 (limited by SFDL grammar)                                                            |
 | `secs4j/Secs4jHostHarness.java`            | secs4java8 — Apache 2.0 Java impl by kenta-shimizu | **55 checks** across S1/S2/S3/S5/S6/S7/S10/S14/S16, including the full E40 body that defeated secsgem-py and unsolicited S6F11/S5F1 observation |
 | `tshark_validate.sh`                       | Wireshark's built-in HSMS dissector         | 69 captured frames dissected with no malformed-packet warnings                                                       |
@@ -125,7 +126,7 @@ is consistent:
    non-Python validators).
 2. Wired into `.gitea/workflows/ci.yml` as a separate job.
 3. Listed in this README's table + in `../VERIFICATION.md`.
-4. Surfaced in [`../PROOFS.md`](../PROOFS.md) if it adds a
+4. Surfaced in [`../docs/PROOFS.md`](../docs/PROOFS.md) if it adds a
    meaningful new dimension.
 
 Bug reports from a new validator → file at