Tracking fws U3: observation registry with read completeness #2018

What this unit delivers: a task-local observation registry that records when a full-file observation of a path exists, with a version token and a completion state. A partial read does not make a destination observable, and a move carries the source's completeness rather than inventing a new observation.

Why it matters: this is the bookkeeping that lets a later guarded write tell "the model actually read this whole file" from "the model saw a slice" - the difference between allowing and refusing a full-file overwrite.

Implemented by: PR #1912 (branch fws/u3-observation-completeness).

Acceptance:

• Registration, versioning, completion and expiry of observations, scoped to one task.
• A partial read never marks a path complete.
• A move re-targets the source's observation onto the destination.
• The registry is not wired into production call sites yet; the units after this one introduce the call sites with their own behaviour tests.

Part of the file-write-safety epic #1375, landed as a nine-PR chain tracked on #1991. Merge order: U1 -> U2 -> U3 -> U4 -> U5 -> U8 -> U6 -> U7 -> U9.