feat: Major update w/new capabilities

- Add typed KV constructors to `doterr.go`: `StringKV()`,
  `IntKV()`, `Int64KV()`, `BoolKV()`, `Float64KV()`, `AnyKV()`,
  `ErrorKV()`.
- Add `MsgErr()` for ad-hoc errors without sentinels (rapid
  development helper).
- Add `AppendKV()` for accumulating metadata with lazy
  evaluation support via `func()KV`.
- Enhance `ErrMeta()` to recursively collect metadata from all
  `doterr` entries in error tree, not just immediate children.
- Add experimental `dtx/` subpackage with `OrderedMap[K,V]`,
  `Null[T]`, and `DirPath` extensions.
- Add `DirPath.Remove()`, `DirPath.IsTildePath()`,
  `PathSegments.Values()`, and `PathSegments.WalkFS()`.
- Fix `DirPath.ToTilde()` to use `TildeDirPathJoin()` for
  correct path construction.
- Add ADR adopting Go Stability package for API stability with
  stability levels `stable`, `provisional`, `experimental`,
  `deprecated`, `obsolete`, and `internal`.
  with deprecation policy documented.
- Update `README.md` with stability policy, contract format,
  objections section, and governance notes.

Author: mikeschinkel