Migrate symbol deep-link syntax from #S:Name to #::Name

Symbol deep-link URL syntax: migrate from #S:Name to #::Name

Background

Muse already owns :: as its universal symbol separator — it appears in every CLI address (muse code cat "domain.py::InsertOp"), every issue anchor, every impact query. The current blob deep-link fragment #S:InsertOp works correctly but borrows a convention from outside the ecosystem rather than using Muse's own idiom.

A developer who knows the CLI should be able to look at:

muse/domain.py#::InsertOp

and immediately read it as "scope into symbol InsertOp" — the same mental model as domain.py::InsertOp in the terminal.

The current #S: prefix also has a minor inconsistency: line anchors use no separator (#L160) while symbol anchors use a colon (#S:InsertOp). #::InsertOp sidesteps this entirely — the :: is the separator, the same as everywhere else in Muse.

Goal

• #::InsertOp deep-links to the InsertOp symbol in a blob view
• #S:InsertOp continues to work as a permanent alias (backward compatibility for any existing links in issues, mists, external docs)
• The canonical form emitted by symbol_anchor.py and the UI anchor picker becomes #::Name

Scope

• musehub/services/symbol_anchor.py — update blob_url() to emit #::Name
• musehub/api/routes/musehub/ui_blob.py — blob page fragment resolution logic
• UI anchor picker — generate #::Name in copy-to-clipboard and insert-link flows
• Docs / README references to #S: syntax

Out of scope

• Changing the :: separator in CLI addresses or issue anchor strings — those are already correct
• Any changes to line anchors (#L160, #L160-L175) — those stay as-is