feat(code-intel): MQL (MQL4/5) + C++ call-graph code intelligence

[immy2good]

What

Adds an MQL-aware extraction layer (.mq4 / .mq5 / .mqh parsed via the
already-shipped tree-sitter-cpp grammar under a distinct mql language tag) plus
C++ receiver-type resolution, producing a precise within-source call graph.

Why

MQL4/MQL5 (MetaTrader) is a C/C++ subset with no dedicated tree-sitter grammar.
Tagging it mql over tree-sitter-cpp gives definitions, call edges, and
code_callers/code_callees/code_blast/code_flow for a large, previously
unsupported language family — and the C++ receiver-resolution work it required is
general-purpose (benefits plain C++ too).

What's in it

Definitions — class methods as scoped chunks (Class.method), inline and
out-of-line (C::method(){...}).

Call edges with receiver-type resolution

• implicit-self (m(), this->m()) → enclosing class (exactly-one-declaration rule)
• out-of-line (C::m(){...}) → class via the definition's qualified_identifier scope
• member-field / local-object (m_x.Method(), obj.Method()) → the receiver's
  declared class type (cross-file; the dominant real-world shape)
• Honest precision boundary: genuine globals/library calls and overloads stay
  bare — no confident false edges.

Short-name recall — getCallersOf/getCalleesOf match a bare symbol against a
qualified edge's last segment, so receiver-resolved edges stay reachable by short
name (honors the documented "bare or qualified name" param). One shared predicate
keeps the pglite + postgres engines byte-identical (engine parity).

Recursive walk — code_blast/code_flow enabled for cpp + mql.

Testing

All TDD. New suites cover defs, every receiver shape, the precision boundary,
short-name recall, recursive walk, and source-scoped multi-source queries. Engine
parity preserved. Validated on a real multi-file MQL corpus: 165 qualified call
edges, 100% resolving to a real def, 0 false edges.

Marked draft pending a final review pass.