Phase 3: Grammar-Kit parser, PSI and ParserDefinition

Adds src/main/grammar/Jai.bnf, the generated parser/PSI in src/main/gen, a
JaiParserDefinition, and the Tier 3 corpus parse gate (406/714 files clean).

Two things were not obvious:

- Grammar-Kit mints its own token instances from the .bnf 'tokens' block, which
  are different objects from the ones JaiLexer emits, so every rule silently
  failed to match. Fixed with tokenTypeFactory -> JaiTokenTypes.byName.
- A backslash inside an identifier is a continuation in the compiler's lexer
  (Jai_Lexer/module.jai:444): 'left\_margin' is one identifier. JaiLexer now
  does the same; the Tier 0 round-trip still holds because the token span
  covers the backslash and the skipped spaces.

Constructs that BNF alone cannot express live in JaiParserUtil: directive-name
tests (#ident is one token), '==' before '{' for the switch form, procedure
header vs parenthesised expression, and the declaration lookahead.

AGENTS.md

@@ -78,6 +78,10 @@ Tier 0: lexed 714 files, 17154195 chars, 3009381 tokens cleanly.
   `~/.local/jai`, keywords/tokens transcribed from the compiler's own lexer.
   **Read this before writing any lexer or parser code.**
 - `docs/BUILD_PLAN.md` — phased plan, testing strategy, risks, open questions.
+- Local IntelliJ reference projects and documentation in `~/programming/thirdparty`:
+  - `fortran-plugin` — a simple language plugin to use as reference.
+  - `intellij-elixir` — a more complex plugin to use as reference.
+  - `intellij-sdk-docs` — local versions of the IntelliJ SDK documentation.
 - `mise.toml` → `java = "temurin-21"` (was 17, which was broken).
 - `jaigradle` — JDK-resolving Gradle wrapper. Executable, verified.
 - `jaitest` — runs the suite and reports the JUnit XML (see above).
@@ -131,9 +135,8 @@ Both are justified in `docs/BUILD_PLAN.md` §2. Do not silently reverse them.
   Nested block comments need a depth counter. Both would end up as hand-written
   Java inside JFlex actions anyway.
 - **Grammar-Kit BNF for parser + PSI.** Verified headless: the plugin ID is
-  `org.jetbrains.intellij.platform.grammarkit`, bundled in the IntelliJ Platform
-  Gradle Plugin since 2.12.0 (we run 2.18.1). `generateLexer` / `generateParser`
-  already appear in `./jaigradle tasks --all` — no extra dependency needed.
+  `org.jetbrains.intellij.platform.grammarkit`, applied in `build.gradle.kts`.
+  `generateLexer` / `generateParser` tasks are present in `./jaigradle tasks`.
 - **Ship highlighting before the parser.** Highlighting needs only the lexer.
 
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