feat(space): wire write path into InformationSpace; integration (WP-0008 T6)

edit()/overlay()/apply_overlay() on InformationSpace. edit() unifies the write
path through one principled route — draft overlay then apply: write-through-capable
target fast-forwards (APPLIED), read-only target keeps the draft as local truth
(KEPT_DRAFT), external drift refuses (no clobber). Integration tests cover all
four. 64 tests green, pyflakes clean. Flips WP-0008 done.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

workplans/SHARD-WP-0008-write-path.md

@@ -4,7 +4,7 @@ type: workplan
 title: "write path — overlay engine, writable adapter, apply-under-drift"
 domain: whynot
 repo: shard-wiki
-status: active
+status: done
 owner: tegwick
 topic_slug: whynot
 created: "2026-06-15"
@@ -114,7 +114,7 @@ overlay. Tests: page with a draft reads with overlay_state DRAFT; applied/none r
 
 ```task
 id: SHARD-WP-0008-T6
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "ab01fffb-61ad-416c-9f13-fdfbfd503153"
 ```