build: the submodule is how libavr arrives; the era already carries it

The pin the whole history now encodes becomes the primary route: the
submodule default replaces FetchContent and the unpinned forge fallback,
LIBAVR_ROOT stays as the tandem-development override, the presets already
take the toolchain file from the submodule, and the Studio projects anchor
their include path there — correct by construction. The version tags and
the one-command historical build are documented beside the version map.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ide/pureboot/pureboot.cppproj

@@ -60,7 +60,7 @@
         </avrgcccpp.compiler.symbols.DefSymbols>
         <avrgcccpp.compiler.directories.IncludePaths>
           <ListValues>
-            <Value>$(MSBuildProjectDirectory)\..\..\..\libavr\include</Value>
+            <Value>$(MSBuildProjectDirectory)\..\..\libavr\include</Value>
           </ListValues>
         </avrgcccpp.compiler.directories.IncludePaths>
         <avrgcccpp.compiler.optimization.level>Optimize for size (-Os)</avrgcccpp.compiler.optimization.level>
@@ -92,7 +92,7 @@
         </avrgcccpp.compiler.symbols.DefSymbols>
         <avrgcccpp.compiler.directories.IncludePaths>
           <ListValues>
-            <Value>$(MSBuildProjectDirectory)\..\..\..\libavr\include</Value>
+            <Value>$(MSBuildProjectDirectory)\..\..\libavr\include</Value>
           </ListValues>
         </avrgcccpp.compiler.directories.IncludePaths>
         <avrgcccpp.compiler.optimization.level>Optimize for size (-Os)</avrgcccpp.compiler.optimization.level>