fix(docs): repoint README soundness-ledger refs to docs/SOUNDNESS.adoc

README referenced `SOUNDNESS-LEDGER.adoc` (a filename that does not exist)
in three places. The soundness single-source-of-truth is `docs/SOUNDNESS.adoc`.

This was not cosmetic: `tools/check-soundness-ledger.sh` Property 2
(back-links) does `grep -q "SOUNDNESS.adoc"` over the pointer docs, and the
old string did not match, so the soundness-ledger gate (wired into
`just check` + CI) was RED:

  ERROR (property 2): README.adoc no longer links to docs/SOUNDNESS.adoc

After the fix the full gate passes (built off system OCaml 4.14.1 +
opam, dune 3.24.0): "OK: soundness ledger — all 5 properties hold".
Docs-only; no build/behaviour change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

README.adoc

@@ -27,7 +27,7 @@ Early and experimental. This is *v0.2*: the architecture is settled, the
 implementation and the metatheory are partial and still moving. It is suitable
 for experimentation, teaching, and small sound components -- not yet for
 production. What is proven, what is implemented, and what is still prose are
-stated plainly below and tracked in `SOUNDNESS-LEDGER.adoc`.
+stated plainly below and tracked in `docs/SOUNDNESS.adoc`.
 
 == What you get
 
@@ -142,7 +142,7 @@ risk here, and it is not yet solved.
 Soundness is *partially mechanised, and honestly tracked.* An initial,
 axiom-free, machine-checked result for code-generation preservation exists
 (Coq/Rocq). A number of residuals remain open; they are recorded -- not hidden
--- in `SOUNDNESS-LEDGER.adoc`, which states for each claim whether it is
+-- in `docs/SOUNDNESS.adoc`, which states for each claim whether it is
 mechanised or still argued in prose.
 
 The ledger is the source of truth for what currently holds. This README
@@ -192,7 +192,7 @@ totality cut, and a WebAssembly target -- rather than any one ingredient.
 == Documentation
 
 // TODO: link the design notes and the examples directory once locations are stable.
-* Soundness status: `SOUNDNESS-LEDGER.adoc`
+* Soundness status: `docs/SOUNDNESS.adoc`
 
 == License