library_revise_docx takes an address in arm B

The edit tool required context_before and context_after -- ~40 characters
either side of the target, retyped by the model from memory of text it read
earlier. Every misquote in the edit benchmark was one of those strings: the
model joined two lines with a space where the document has a newline, or
invented a blank line where extraction has one.

With `at`, the model sends only the provision and what to change. The server
locates `find` inside that provision's span and reads the surrounding
characters off the document, so the bytes come from the document instead of
from the model's reconstruction of it.

Legacy keeps the old required shape. Arm B drops the context pair from
`required` and adds `at`.

Also hoisted NAV_TOOL_SHAPE above its first use -- the tool factory runs at
module init and read it from the temporal dead zone, which broke 13 tests.

1,195 tests pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9ToHYJVDxfeJcJwdzrP2H