feat(datamap): write msgpack via ant_core::data::write_datamap

[Nic-dorman]

Summary

Closes the ant-gui → ant-cli round-trip gap. The read side already delegates to `ant_core::data::read_datamap` (sniffs first byte: msgpack canonical, JSON legacy), but the write side was still on the local JSON path — so a datamap produced by ant-gui couldn't be opened by `ant file download --datamap` (msgpack-only).

After this PR, ant-gui uploads produce `..datamap` (e.g. `holiday.jpg.datamap`), msgpack-encoded, and read back cleanly in any consumer of `ant_core::data`.

Changes

• `src-tauri/src/config.rs` — `write_datamap_for` now takes `&DataMap` and delegates to `ant_core::data::write_datamap`. Drops the local sanitize-stem helper, the `MAX_COLLISION_ATTEMPTS` loop, the `DATAMAP_EXTENSION` constant, and the `std::io::Write` / `std::path::Path` imports. Upstream handles sanitization, atomic tempfile-and-rename, parent-dir fsync on Unix, and the `NumericSuffix` collision policy.
• `src-tauri/src/autonomi_ops.rs` — three call sites in `confirm_upload`, `confirm_upload_merkle`, and `wallet_upload` pass `&result.data_map` instead of the JSON string. The `data_map_json` re-encoding and SHA-derived `address` stay; only the on-disk encoding moves.

Net diff: -47 lines.

Compatibility

Existing `upload_history.json` entries reference JSON datamaps on disk via their `data_map_file` path. Those keep working — `read_datamap_file` already accepts both formats. New uploads write msgpack with the extension-preserved naming. Coexistence is intentional; no migration is needed.

Test plan

• Build clean: `cargo check` passes (verified locally — 0 warnings)
• CI vitest (no JS changes in this PR; existing tests should pass)
• Manual: upload a small file (e.g. `test.pdf`). Confirm `%APPDATA%\autonomi\ant-gui\test.pdf.datamap` exists, content's first byte is not `0x7B` (`{`).
• Manual: open the same datamap via Download by Datamap inside ant-gui → should download successfully.
• Manual round-trip: copy the new `test.pdf.datamap` to a machine with `ant-cli`, run `ant file download --datamap test.pdf.datamap` → should succeed (this is the gap this PR closes).
• Regression: download an existing entry from before this PR (the old `.datamap` JSON files) → should still work.
• Collision: upload two files named `test.pdf` in succession; second produces `test.pdf-2.datamap` (or upstream's actual suffix scheme), not an overwrite.

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