feat: add $message expression and payload source (Arazzo 1.1.0)

[char0n]

Summary

Aligns the grammar with the released Arazzo 1.1.0 runtime expression ABNF. The initial 1.1.0 work (#116) inlined the reference grammars and added $self / JSON-Pointer support, but omitted two constructs that are part of the published 1.1.0 grammar:

• $message.{source} expression — for event-driven (AsyncAPI) operations, mirroring $request. / $response.
• payload-reference ("payload" ["#" json-pointer]) — a new source alternative alongside header / query / path / body, enabling $message.payload#/...

After this change, the grammar matches the 1.1.0 runtime-expression ABNF exactly.

Changes

• src/grammar.bnf — add $message. alternative and payload-reference rule (+ recompiled src/grammar.js)
• src/parse/translators/CSTTranslator.js — register payload-reference callback
• src/parse/translators/ASTTranslator/transformers.js — add MessageExpression and PayloadReference transformers
• types/index.d.ts — add MessageExpression, PayloadReference; extend Reference and ASTNode unions
• README.md — sync grammar block and add a $message.payload example row
• test/fixtures/expressions-valid.js, test/test.js — new fixtures/assertions for $message (header/query/path/body/payload, with and without JSON Pointer)

Test plan

• All 169 tests pass
• New CST/AST corpus snapshots generated for the $message/payload fixtures
• Runtime smoke test: $message.payload#/user/id parses to MessageExpression → Source → PayloadReference → JsonPointer; embedded extraction {$message.payload#/user/id} works; invalid forms ($message., {/} in pointer) correctly rejected
• No new tsc errors introduced (the two pre-existing types/index.d.ts errors are unrelated and present on main)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates the Arazzo runtime-expression grammar and AST/CST translators to support Arazzo 1.1.0’s $message.{source} expressions and the new payload-reference source (e.g., $message.payload#/...), and extends types/docs/tests accordingly.

Changes:

• Extend the grammar ($message. + payload-reference) and regenerate the compiled grammar output.
• Add CST/AST translation support for payload-reference and MessageExpression.
• Expand TypeScript typings, fixtures, and snapshot coverage; update README examples.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/grammar.bnf	Adds $message. expression alternative and payload-reference rule to match the 1.1.0 ABNF.
src/grammar.js	Regenerated compiled grammar reflecting the updated ABNF.
src/parse/translators/CSTTranslator.js	Registers payload-reference CST callback for tree generation.
src/parse/translators/ASTTranslator/transformers.js	Adds MessageExpression and PayloadReference AST transformations.
types/index.d.ts	Adds MessageExpression/PayloadReference types and extends unions to include them.
README.md	Syncs the documented ABNF block and adds a $message.payload example row.
test/fixtures/expressions-valid.js	Adds valid expression fixtures for $message.* and payload forms.
test/test.js	Adds expression-detection assertions for $message and $self.
test/parse/__snapshots__/cst-corpus.js.snap	Updates CST snapshots for newly supported $message expressions.
test/parse/__snapshots__/ast-corpus.js.snap	Updates AST snapshots for newly supported $message expressions and payload references.

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[char0n]

🎉 This PR is included in version 3.0.0 🎉

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