docs: the storage rationale lives with the architecture it justifies

storage-stack-discussion.md was misplaced at the docs root — that level is
for track plans; this is the file-system domain's design record. Moved to
file-system-development/storage-design-rationale.md, renamed to say what it
is, cross-references updated.

docs/file-system-development/storage-architecture.md

@@ -1,7 +1,7 @@
 # The storage architecture: layers, boundaries, responsibilities
 
 > **Status:** the layered model below is the settled design
-> ([storage-stack-discussion.md](../storage-stack-discussion.md) records how it
+> ([storage-design-rationale.md](storage-design-rationale.md) records how it
 > was reached and what the surveyed systems taught). The data path — vfs
 > protocol, kernel mount routing, the FAT service, the block protocol,
 > usb-storage — is **built**. The volume manager, the driver's range

docs/file-system-development/storage-design-rationale.md

@@ -1,12 +1,16 @@
-# The storage stack: block, volumes, filesystems — a discussion
+# The storage design rationale: why the stack is shaped this way
 
-*2026-08-09. Design discussion, not a plan. The questions, verbatim: should the
+*2026-08-09. The design record behind
-block protocol be separate from the VFS? how do channels work with these block
+[storage-architecture.md](storage-architecture.md): the survey, the
-devices? how should we wire up different filesystems? Grounded in a survey of how
+trade-offs, and the decisions with their reasons — kept so future changes
-Minix 3, QNX Neutrino, Fuchsia, Redox, Plan 9, and Linux each answered the same
+argue against the evidence rather than rediscovering it. The questions that
-questions, and in exactly where our own seams sit today. The layering rule this
+drove it, verbatim: should the block protocol be separate from the VFS? how
-discussion serves: drivers are the lowest level (hardware only); VFS and the
+do channels work with these block devices? how should we wire up different
-filesystems are higher layers; the protocol layer routes between them.*
+filesystems? Grounded in how Minix 3, QNX Neutrino, Fuchsia, Redox, Plan 9,
+and Linux each answered the same questions, and in exactly where our own
+seams sat. The layering rule it serves: drivers are the lowest level
+(hardware only); VFS and the filesystems are higher layers; the protocol
+layer routes between them.*
 
 ## What the survey says, compressed