chore: bump Motoko to v1.15.0 (#373)

## Summary

Automated bump of `.sources/motoko` from `v1.14.1` to `v1.15.0`.

- Ran `npm run sync:motoko` — synced docs from the new release
- Build passed ✓

## Checklist

- [ ] Review synced content for breaking changes
- [ ] Check [release
notes](https://github.com/caffeinelabs/motoko/releases/tag/1.15.0) for
API or syntax changes that affect hand-written docs
- [ ] Verify any new sections or renamed files are handled correctly

## Sync recommendation

`sync from caffeinelabs/motoko doc/md`

Co-authored-by: pr-automation-bot-public[bot] <pr-automation-bot-public[bot]@users.noreply.github.com>

Author: pr-automation-bot-public[bot]

.sources/VERSIONS

@@ -38,5 +38,5 @@
 # appear in docs before they ship to users.
 # -------------------------------------------------------
 
-motoko                v1.14.1                                                   58c3239
+motoko                v1.15.0                                                     0cc90c1
 internetidentity      release-2026-08-21                                        4c934d1f

.sources/motoko

@@ -8,6 +8,29 @@ sidebar:
 
 # Motoko compiler changelog
 
+## 1.15.0 (2026-08-28)
+
+* motoko (`moc`)
+
+  * feat: with `--stable-baseline` and `--enhanced-migration`, the migration
+    directory is now validated against the migration history the baseline
+    records as already applied: a deployed migration that was deleted (unless
+    all older ones are deleted too), edited in place, or a local migration
+    backdated to sort before the deployed head reports the new M0268
+    diagnostic, a warning treated as an error by default (demote with
+    `-W=M0268`, silence with `-A=M0268`) (#6325).
+
+  * bugfix: with `--stable-baseline` and `--enhanced-migration`, the M0254/M0267
+    check now honors the migrations the baseline records as already applied:
+    requirements are computed at the chain's resume point instead of replaying
+    the whole chain, and fields the baseline explains no longer warn M0254;
+    a field missing from the baseline errors with M0267 naming the resume
+    point, while an incompatible one keeps the detailed M0170/M0216
+    compatibility errors. Each problem is reported once, and the
+    write-a-migration hint is only offered when adding a migration file can
+    actually fix the field (#6318).
+  * bugfix: fixes compilation error on <system>-enabled mixin (#6328).
+
 ## 1.14.1 (2026-08-17)
 
 * motoko (`moc`)