fix(docs): README back-links + real LICENSE section (unblocks 2 red CI gates) (#677)

## Problem

README failed **two** CI doc-governance gates (both in
`.github/workflows/ci.yml`):

1. **`check-soundness-ledger.sh`** (property 2, back-links) — README
pointed at `SOUNDNESS-LEDGER.adoc`, a filename that **does not exist**.
The gate greps for `SOUNDNESS.adoc`; the old string didn't match → RED.
2. **`check-doc-truthing.sh`** (DOC-04) — README did **not** mention the
authoritative status matrix `CAPABILITY-MATRIX.adoc` anywhere → RED.

Both were pre-existing reds on `main` (verified against pristine
`origin/main`).

## Fix (README.adoc only)

- Repoint the 3 soundness-ledger references → `docs/SOUNDNESS.adoc`
- Add `docs/CAPABILITY-MATRIX.adoc` (feature readiness) in Status +
Documentation
- Replace the stale *"state the license and add a LICENSE file"* TODO
with a real License section (`LICENSE` + `LICENSES/` already exist:
MPL-2.0 code, CC-BY-SA-4.0 docs)

## Verification (real toolchain — dune 3.24.0, OCaml 4.14.1 + opam)

```
BUILD_EXIT=0
OK: doc-truthing intact — presence invariants + over-claim ratchet (DOC-04/05/08/09).  (TRUTH_EXIT=0)
OK: soundness ledger — all 5 properties hold.                                          (GATE_EXIT=0)
```

## Supersedes #675

#675 fixed only the soundness link, so it could not turn the
doc-truthing gate green on its own (that gate is required, so #675 was
unmergeable alone). This PR fixes both. **#675 will be closed as
superseded.** Also unblocks the descriptiles migration #676 (blocked
only by these same README gates).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: hyperpolymath

README.adoc

@@ -27,7 +27,8 @@ 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`; per-feature
+readiness is tracked in `docs/CAPABILITY-MATRIX.adoc`.
 
 == What you get
 
@@ -142,7 +143,7 @@ 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
@@ -192,8 +193,12 @@ 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`
+* Feature readiness: `docs/CAPABILITY-MATRIX.adoc`
 
 == License
 
-// TODO: state the license and add a LICENSE file.
+Code is licensed under the Mozilla Public License 2.0 (`MPL-2.0`); prose and
+documentation under Creative Commons Attribution-ShareAlike 4.0
+(`CC-BY-SA-4.0`). Full texts are in `LICENSE` and `LICENSES/`; per-file
+provenance is declared with SPDX headers.