… drop plan from PR

- Add universal_io/multi_universal_read.rs with SourceId, MultiUniversalRead,
  VecMultiUniversalRead and test; re-export from mod.rs.
- Remove docs/design/multi-file-universal-io-plan.md from the branch.

Made-with: Cursor

…tach, len, is_empty to trait

- MultiUniversalRead: remove default impls for populate() and clear_ram_cache();
  they are now required.
- Add to trait: new(), len(), is_empty() (default), attach() (default Err).
- Introduce associated type Source for attach; add AttachUnsupported<T>
  placeholder for impls that do not support dynamic attach.
- VecMultiUniversalRead: type Source = S; implement all trait methods.
- Re-export AttachUnsupported from universal_io.

Made-with: Cursor

…all impls

- Drop AttachUnsupported placeholder type and its UniversalRead/Send impls.
- Make attach() a required method on MultiUniversalRead (no default).
- Doc: all implementations must support attaching sources dynamically.
- Remove AttachUnsupported from re-exports.

Made-with: Cursor

…, add MultiUniversalWrite

MultiUniversalRead:
- Remove type Source; attach(path, options) opens by path and returns SourceId.
- new(options: OpenOptions) for creating an empty multi-source view.
- Move VecMultiUniversalRead to vec_multi_universal_read.rs.

MultiUniversalWrite (new):
- Trait: new(options), len(), is_empty(), attach(path, options),
  write_batch_multi((SourceId, offset, data)...), source_len, flusher(),
  populate(), clear_ram_cache().
- VecMultiUniversalWrite in vec_multi_universal_write.rs; flusher()
  runs all source flushers.

Re-export MultiUniversalWrite, VecMultiUniversalWrite from universal_io.

Made-with: Cursor

Make MultiUniversalWrite<T>: MultiUniversalRead<T> like UniversalWrite
extends UniversalRead. Remove duplicated methods (new, len, is_empty,
attach, source_len, populate, clear_ram_cache) from the write trait;
keep only write_batch_multi and flusher. VecMultiUniversalWrite now
impl MultiUniversalRead and MultiUniversalWrite separately.

Made-with: Cursor