feat(asm): Phase A.0 — add kw_asm keyword + lex test

`asm` now lexes as a dedicated `kw_asm` keyword (Token.Tag + keyword map entry).
`volatile` and `clobbers` stay out of the global keyword table — they are
recognized contextually only inside an `asm { … }` body (PLAN-ASM Deviation 4).

- token.zig: kw_asm tag + `.{ "asm", .kw_asm }` map entry.
- lsp/server.zig: classifyToken exhaustive switch gained the .kw_asm arm
  (the new enum value forced coverage — intended tripwire).
- lexer.test.zig (new, wired into root.zig barrel): locks `asm`->kw_asm and
  `volatile`/`clobbers`->identifier.

Lock commit (behavior-locking passing test). zig build test green (445 unit).

src/token.zig

@@ -45,6 +45,7 @@ pub const Tag = enum {
     kw_callconv, // callconv (calling convention annotation)
     kw_extern, // extern (import: external linkage, C ABI, no body)
     kw_export, // export (define + expose: external linkage, C ABI)
+    kw_asm, // asm (inline assembly expression / global asm decl)
 
     // Symbols
     colon, // :
@@ -283,6 +284,9 @@ pub const keywords = std.StaticStringMap(Tag).initComptime(.{
     .{ "callconv", .kw_callconv },
     .{ "extern", .kw_extern },
     .{ "export", .kw_export },
+    // `asm` is a real keyword; `volatile` / `clobbers` stay OUT of this table
+    // (recognized contextually only inside an `asm { … }` body — see PLAN-ASM).
+    .{ "asm", .kw_asm },
 });
 
 pub fn getKeyword(bytes: []const u8) ?Tag {