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/debug/Dwarf.zig

@@ -2247,7 +2247,7 @@ pub const ElfModule = struct {
                 var decompress: std.compress.flate.Decompress = .init(&section_reader, .zlib, &.{});
                 var decompressed_section: ArrayList(u8) = .empty;
                 defer decompressed_section.deinit(gpa);
-                decompress.reader.appendRemainingUnlimited(gpa, null, &decompressed_section, std.compress.flate.history_len) catch {
+                decompress.reader.appendRemainingUnlimited(gpa, &decompressed_section) catch {
                     invalidDebugInfoDetected();
                     continue;
                 };