[datlechin]

Summary

• Saved query SQL takes tab stops: ${1:default}, ${1}, ${1|ASC,DESC|} and ${0}. The same number twice is one field, mirrored as you type.
• After keyword expansion, Tab and Shift+Tab move between fields with the field's text selected. Escape, or a selection outside the inserted SQL, ends the session.
• A choice field opens a menu at the field.
• ;; still works and means ${0}. A bare $1, $$ or $tag$ stays literal. A field left open leaves the whole query as written.
• Placeholders in saved queries imported from Sequel Ace now expand as fields.
• A completion accepted inside a mirrored field goes into every copy.
• Edits made by the typing filters (auto-pairing) are tracked, so later fields stay in place.
• An unfilled outline emphasis no longer draws a black copy of its text over the syntax colors.
• The saved query dialog footer names fields as well as ;;.

Not in this PR

• Starter snippets per dialect: needs a decision on where they live (seeded saved queries vs a built-in catalog), whether they sync, and the editing UI.
• Driver statement completions as snippets (HanaMetadata.swift:43): a registry plugin that writes ${1:SCHEMA} would insert it literally on hosts without this engine, so it needs a host version gate first.
• Insert in Editor, drag and Open Quickly still insert the raw SQL (MainContentCoordinator+Favorites.swift:15 appends the text to the tab). ;; never worked there either.
• iOS keeps ;; only (SQLCompletionInsertion.resolve).
• Mirrors follow typing through the editor's multiple selections. Two empty copies at the same spot (${1}${1}, or adjacent copies emptied together) collapse into one caret, so typing fills one copy.

Test plan

• SQLSnippetTests (TableProAutocompleteTests): fields, mirrors, choices, nesting, escapes, $1/$$/$tag$ literal, ;; as the final stop, the nesting and expansion limits.
• SnippetSessionTests, SnippetEditingTests (TableProEditorKitTests): Tab visits three fields in order then the end, Shift-Tab, typing into a mirror, adjacent copies, a nested field's edge, completion into mirrors and its undo, a filter edit, Escape, a click outside, choices, undo of the expansion, document replace.
• EmphasisManagerTests.anUnfilledOutlineDrawsNoCopyOfItsText.
• QueryCompletionAdapterLifecycleTests: accepting a saved query with fields selects the first field and keeps $1; ;; alone places the caret as before.

Fixes #3612