✨ feat(mq-lang): add value as name variable binding syntax

[harehare]

Implements jq-style expr as name binding across all language layers

[Copilot]

Pull request overview

Adds jq-style expr as name bindings to the mq language, threading the new syntax through lexing, CST/AST parsing, evaluation, HIR lowering, and formatting so bindings can be introduced mid-pipeline and referenced later.

Changes:

• Introduces as as a keyword/token and parses it into new CST/AST nodes (NodeKind::As / Expr::As).
• Implements evaluation semantics for as bindings by defining the name in the current environment while passing through the prior pipeline value.
• Extends HIR lowering and the formatter to understand/print as bindings, and adds integration/formatter/parser tests.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crates/mq-lang/tests/property_based_tests.rs	Excludes as from generated identifiers to avoid keyword collisions.
crates/mq-lang/tests/integration_tests.rs	Adds eval-level integration coverage for expr as name usage in pipelines and defs.
crates/mq-lang/src/macro_expand.rs	Supports macro expansion/substitution for the new Expr::As node.
crates/mq-lang/src/lexer/token.rs	Adds TokenKind::As and display formatting.
crates/mq-lang/src/lexer.rs	Recognizes as as a keyword during lexing.
crates/mq-lang/src/eval.rs	Evaluates Expr::As by binding the evaluated value into the environment and returning the incoming pipeline value.
crates/mq-lang/src/cst/parser.rs	Parses as bindings into CST (NodeKind::As).
crates/mq-lang/src/cst/node.rs	Adds As to CST node kinds.
crates/mq-lang/src/ast/parser.rs	Parses as bindings into AST (Expr::As) and adds a unit test.
crates/mq-lang/src/ast/node.rs	Adds Expr::As and ensures range calculation covers the inner node consistently with similar constructs.
crates/mq-lang/src/ast/code.rs	Adds code formatting output for Expr::As.
crates/mq-hir/src/hir/lower.rs	Lowers CST As into HIR symbols (keyword + variable + initializer relationship).
crates/mq-formatter/src/formatter.rs	Formats as bindings and adds formatter tests.

[codspeed-hq]

Merging this PR will not alter performance

✅ 29 untouched benchmarks

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_{Comparing feat/as-binding (fcf33ba) with main (5d610fe)}