feat(code-action): add inherit to explicit binding conversion action

[takeokunn]

Summary

This PR adds a new "Convert to explicit binding" code action that converts inherit statements to explicit bindings:

• Simple inherit: { inherit x; } → { x = x; }
• Inherit from expression: { inherit (b) a; } → { a = b.a; }

Before	After
{ inherit x; }	{ x = x; }
{ inherit (b) a; }	{ a = b.a; }
{ inherit (foo.bar) baz; }	{ baz = foo.bar.baz; }

Changes

File	Description
nixd/lib/Controller/CodeActions/InheritToBinding.h	Public interface declaration
nixd/lib/Controller/CodeActions/InheritToBinding.cpp	Core implementation
nixd/lib/Controller/CodeAction.cpp	Integration with dispatcher
nixd/lib/meson.build	Build system integration

Behavior

The action is offered when:

• Cursor is on an inherit statement
• The inherit has exactly one name (single-name only)

Limitations:

• Multi-name inherits (inherit x y z;) not supported
• Only static (non-interpolated) attribute names handled

Test Plan

• CI passes
  • 5 regression tests:
    • basic.md: Simple inherit (inherit x → x = x)
    • with-expr.md: Inherit from variable (inherit (b) a → a = b.a)
    • complex-expr.md: Nested expression (inherit (foo.bar) baz)
    • long-name.md: Long attribute names
    • multi-name-negative.md: Multi-name inherits do NOT offer action

Related

• Issue: nixd: code actions #466
• Draft PR: [DRAFT] feat(code-action): add refactoring and quickfix actions for Nix code #755 (this implements Split 9)