std.compress.zstd.Decompress fixes

* std.Io.Reader: appendRemaining no longer supports alignment and has
  different rules about how exceeding limit. Fixed bug where it would
  return success instead of error.StreamTooLong like it was supposed to.

* std.Io.Reader: simplify appendRemaining and appendRemainingUnlimited
  to be implemented based on std.Io.Writer.Allocating

* std.Io.Writer: introduce unreachableRebase

* std.Io.Writer: remove minimum_unused_capacity from Allocating. maybe
  that flexibility could have been handy, but let's see if anyone
  actually needs it. The field is redundant with the superlinear growth
  of ArrayList capacity.

* std.Io.Writer: growingRebase also ensures total capacity on the
  preserve parameter, making it no longer necessary to do
  ensureTotalCapacity at the usage site of decompression streams.

* std.compress.flate.Decompress: fix rebase not taking into account seek

* std.compress.zstd.Decompress: split into "direct" and "indirect" usage
  patterns depending on whether a buffer is provided to init, matching
  how flate works. Remove some overzealous asserts that prevented buffer
  expansion from within rebase implementation.

* std.zig: fix readSourceFileToAlloc returning an overaligned slice
  which was difficult to free correctly.

fixes #24608

lib/std/array_list.zig

@@ -1033,7 +1033,7 @@ pub fn Aligned(comptime T: type, comptime alignment: ?mem.Alignment) type {
         pub fn print(self: *Self, gpa: Allocator, comptime fmt: []const u8, args: anytype) error{OutOfMemory}!void {
             comptime assert(T == u8);
             try self.ensureUnusedCapacity(gpa, fmt.len);
-            var aw: std.io.Writer.Allocating = .fromArrayList(gpa, self);
+            var aw: std.Io.Writer.Allocating = .fromArrayList(gpa, self);
             defer self.* = aw.toArrayList();
             return aw.writer.print(fmt, args) catch |err| switch (err) {
                 error.WriteFailed => return error.OutOfMemory,