docs: one documentation directory, not two (#137)

docs/ has held the README's images since the fair-launch release. I
added doc/ yesterday for the derivation spec without looking, and #108
then correctly used docs/ -- leaving the repository with both, one
letter apart, which is the kind of thing that quietly costs someone ten
minutes a year forever.

The mistake was mine, so the move is mine: derivation.md and
verify-derivation.py go to docs/, and the two README links follow.

No content changes. The verifier still reproduces the wallet from its
new path, which is the only thing here that can actually break.

3 files changed, 3 insertions, 3 deletions

diff --git a/README.md b/README.md
index 6446b3d..6746e02 100644
--- a/README.md
+++ b/README.md
@@ -107,7 +107,7 @@ words also cover the address the window shows you. **Both still matter** — the
 phrase and the file back up different things, and the difference is where people
 lose money.
 
-The derivation is written down in [doc/derivation.md](doc/derivation.md), with
+The derivation is written down in [docs/derivation.md](docs/derivation.md), with
 test vectors and a script that reproduces them from scratch. It is there so the
 twelve words keep working even if this software does not: paths, address format
 and encoding, enough to recover the keys with ordinary tools and no Bitflash
diff --git a/doc/derivation.md b/docs/derivation.md
index 3ca8339..c5ee3ad 100644
--- a/doc/derivation.md
+++ b/docs/derivation.md
@@ -5,7 +5,7 @@ software**. If the project stops, the binaries are lost, or you simply do not
 trust them, the twelve words plus what is written here are enough to recover
 your private keys with ordinary cryptographic tools.
 
-Everything below is verified by `doc/verify-derivation.py`, a self-contained
+Everything below is verified by `docs/verify-derivation.py`, a self-contained
 script that implements this document from scratch — no Bitflash code, no
 libraries beyond Python's standard library — and checks that it reproduces the
 addresses the wallet produces. If the two ever disagree, this document is wrong
@@ -106,7 +106,7 @@ abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon
 Check them yourself:
 
 ```
-python doc/verify-derivation.py
+python docs/verify-derivation.py
 ```
 
 Both tables were produced by the wallet — the legacy one by the v1.2.15 binary,
diff --git a/doc/verify-derivation.py b/docs/verify-derivation.py
index ba8e883..ba8e883 100644
--- a/doc/verify-derivation.py
+++ b/docs/verify-derivation.py