D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/core/ext4d/source/ext4-blockdev/Cargo.toml

@@ -7,8 +7,8 @@ license.workspace = true
 
 [dependencies]
 rsext4.workspace = true
-redox_syscall = { workspace = true, optional = true }
-libredox = { workspace = true, optional = true }
+redox_syscall = { path = "../../../../../../local/sources/syscall", workspace = true, optional = true }
+libredox = { path = "../../../../../../local/sources/libredox", workspace = true, optional = true }
 log.workspace = true
 
 [features]