Non-bare identifiers: deepen autocomplete matching + EXPLAIN-AST/MV-target lineage #44

Follow-up from the #43 code review. The primary identifier-quoting fix (#43) makes the schema tree and lineage-graph vectors emit valid SQL for names like target_all.part-00000-…-c000.snappy.parquet``. Two deeper paths remain shallow and are tracked here.

1. Editor autocomplete can't match/qualify non-bare names

completions.js now quotes the candidate insert value, but completionContext (same file) still detects the typed word and the qualified parent using [A-Za-z0-9_] only:

• Mid-word splice: typing part-00 leaves the caret in a word that starts after the - (word = 00); accepting splices the quoted name into the middle → part-`` part-…parquet` `` — invalid. (Pre-existing for dashed names; quoting doesn't fix it.)
• Lost qualified completion: after a backticked parent `my-db`., the char before the . is a backtick, so parent='' and the table's column dropdown never appears.

A real fix teaches completionContext to recognize a backtick-quoted run as a single word/parent, and to match candidates whose label contains non-word chars.

2. qualify() is a dot-heuristic for MV-target / EXPLAIN-AST names

schema-graph.js qualify(db, name) decides "already db-qualified?" by name.includes('.'). For a dotted table name this misfires:

• parseMvTarget won't match a backticked TO target → no writes edge.
• parseAstTables tokens for dotted/backticked names are treated as already-qualified, so byId.has(sid) (keyed on raw db.name) misses → feeds/reads edges dropped.

So whole-DB/table lineage silently omits edges for exactly the dotted (parquet-import) tables that motivated #43. A depth-correct fix carries db separately out of parseMvTarget/parseAstTables (and strips backticks) instead of guessing from a dot.

3. (Optional) No chokepoint / guard for identifier quoting

Quoting is applied at ~8 call sites with no single constructor and no test/lint that flags a future raw '… FROM ' + name. Consider a guard test or a structural chokepoint so a new identifier→SQL site can't silently reintroduce the bug.

Context: #43 (the quoting fix), #41 (the lineage graph).