grammar: accept original SiSU bespoke @key: header dialectHEADmain

Until now the grammar handled only the sisudoc-spine YAML header form.
Real SiSU markup uses two textually disjoint header dialects with an
identical body grammar:

  bespoke (original SiSU, ruby):    yaml (sisudoc-spine):
    % SiSU 4.0.0                      # SiSU 8.0
    @title: Alice's Adventures        title: "Alice's Adventures"
    @creator:                         creator:
     :author: Carroll, Lewis            author: "Carroll, Lewis"

Add bespoke-dialect rules alongside the existing yaml ones; the body
grammar is shared between the two.

grammar.js:
- version_comment widened from /# SiSU(spine)? <ver>/ to
  /[#%] SiSU[^\n]*\n/. Real banners observed across both corpora
  include "# SiSU 8.0", "# SiSUspine 8.0", "# SiSU master 8.0",
  "# SiSU: http://...", "% SiSU 4.0.0", "% SiSU 0.72",
  "% SiSU http://...", "% SiSU markup for 0.16 and later".
- Added sisu_header_field / sisu_header_key / sisu_header_value /
  sisu_header_continuation.
- sisu_header_key whitelists the 14 @keys observed in the sisu
  corpus (including the @links:+ additive variant), parallel to
  the existing yaml header_key whitelist.
- sisu_header_continuation accepts any 1+ space indented line whose
  first non-space character is not a newline. Covers " :sub: val",
  " { text }url" freeform under @links:, and 3+ space wrap-line
  continuations (10690 occurrences across the sisu corpus, almost
  all inside @classify: :topic_register: entries).
- Wired sisu_header_field into _toplevel alongside header_field.

queries/highlights.scm:
- Added @keyword / @string captures for the new sisu_* nodes,
  parallel to the existing yaml header captures.

test/corpus/headers_sisu.txt:
- 12 new cases: % SiSU banner variants, @title: with inline value,
  @creator: + :author:, @date: with multiple sub-keys, @make:
  mixed sub-keys, @links: with freeform { text }url continuations,
  @links:+ additive, full bespoke header block, and a coexistence
  case confirming yaml + bespoke at the same top level.

README.md and sisu-markup_tree-sitter.md:
- Describe dual-dialect support; add sisu corpus results table.

Test results:
- tree-sitter test: 79 / 79 pass.
- sisu-markup-samples/data/samples/ (full sisu corpus): 44 / 65
  parse cleanly (was 0 / 65). current/ layout parses at 20 / 21
  (95 %); the dominant failure mode is the wrapped/ layout
  (7 / 21) which trips the pre-existing one-line-per-paragraph
  limitation, not the new header rules.
- sisudoc-spine-samples/markup/ (full spine corpus): 37 / 46
  unchanged. No regression in the yaml dialect.

Mixing the two dialects inside one document remains parseable but
non-idiomatic; enforcement is left to a future linter pass rather
than the grammar.

(assisted by Claude-Code)

1 files changed, 68 insertions, 2 deletions

diff --git a/grammar.js b/grammar.js
index ffaadc2..aa94aad 100644
--- a/grammar.js
+++ b/grammar.js
@@ -33,10 +33,12 @@ module.exports = grammar({
 
     _toplevel: $ => choice(
       $._blank_line,
-      // Header elements
+      // Header elements (yaml dialect, sisudoc-spine)
       $.version_comment,
       $.header_field,
       $.header_comment,
+      // Header elements (bespoke dialect, original SiSU)
+      $.sisu_header_field,
       // Structural headings
       $.heading,
       // Block elements
@@ -70,8 +72,15 @@ module.exports = grammar({
 
     // =================================================================
     // Version comment - first line indicator
+    //
+    // Banner forms observed in the wild span both dialects:
+    //   # SiSU 8.0          # SiSUspine 8.0       # SiSU master 8.0
+    //   % SiSU 4.0.0        % SiSU 0.72           % SiSU 7.1
+    //   # SiSU: http://...  % SiSU markup for 0.16 and later
+    // The prefix is %  (sisu, original) or #  (sisudoc-spine, yaml-era).
+    // Trailing content is free-form (version number, URL, prose).
     // =================================================================
-    version_comment: $ => token(prec(20, /# SiSU(spine)? [0-9]+(\.[0-9]+)*\n/)),
+    version_comment: $ => token(prec(20, /[#%] SiSU[^\n]*\n/)),
 
     // =================================================================
     // Header section (YAML-like metadata before :A~)
@@ -108,6 +117,63 @@ module.exports = grammar({
     header_continuation: $ => token(prec(15, /  [^\n]+\n/)),
 
     // =================================================================
+    // Bespoke header section (original SiSU @key: / :subkey: form)
+    //
+    // Distinct from the yaml header above:
+    //   @creator:                <- sisu_header_key   (starts with '@')
+    //    :author: Carroll, Lewis <- sisu_header_continuation (1-space indent)
+    // vs the yaml form:
+    //   creator:                 <- header_key
+    //     author: "Carroll"      <- header_continuation     (2-space indent)
+    //
+    // The two are textually disjoint so they can coexist at the document
+    // top level without ambiguity. Mixing them inside a single document is
+    // not enforced by the grammar; the markup remains parseable, but
+    // linters (sisu / spine themselves, or an editor diagnostic) are the
+    // right place to flag dialect mixing.
+    // =================================================================
+    sisu_header_field: $ => prec(15, seq(
+      field('key', $.sisu_header_key),
+      optional(field('value', $.sisu_header_value)),
+      '\n',
+      repeat($.sisu_header_continuation),
+    )),
+
+    // Whitelist of top-level @keys observed across the sisu sample
+    // corpus. Restricted to avoid false matches on body @-references
+    // (which lack the trailing colon anyway, e.g. ':A~ @title @creator').
+    sisu_header_key: $ => token(prec(15, choice(
+      '@title:',
+      '@creator:',
+      '@date:',
+      '@rights:',
+      '@classify:',
+      '@identifier:',
+      '@original:',
+      '@notes:',
+      '@links:',
+      '@links:+',        // additive variant seen in samples
+      '@make:',
+      '@publisher:',
+      '@language:',
+      '@vocabulary:',
+    ))),
+
+    sisu_header_value: $ => /[ \t]+[^\n]*/,
+
+    // Continuation line: any indented (1+ leading space) line whose first
+    // non-space character is not a newline. Covers:
+    //   :author: Carroll, Lewis              (1-space sub-key form)
+    //   { Wikipedia }http://...              (1-space freeform under @links:)
+    //   topic_register: SiSU sample:book;    (1-space sub-key form)
+    //     continuation of long value         (3-space wrap-line under sub-key)
+    // The 2-space case overlaps with the yaml header_continuation token,
+    // but disambiguation is contextual: the LR state inside a yaml
+    // header_field expects header_continuation, and the state inside a
+    // sisu_header_field expects sisu_header_continuation.
+    sisu_header_continuation: $ => token(prec(15, / +[^ \n][^\n]*\n/)),
+
+    // =================================================================
     // Headings
     // =================================================================
     heading: $ => prec(18, choice(