phase 3: vimwiki lexer

Two-pass, hand-rolled lexer per SPEC.md §6.6. Block pass walks the
source line by line; recognised constructs emit structural tokens, and
text-bearing line content is fed to the inline pass. Multi-line fences
(`{{{ }}}`, `{{$ }}$`, `%%+ +%%`) flip a `BlockMode` so subsequent
lines are captured raw until the matching closer.

VimwikiToken covers the full §9 feature checklist:

- headings (1–6, centered), horizontal rule, blockquotes (`>` and
  4-space), all list-marker variants, checkboxes, definition `::`,
  tables (separators, header row, col/row span)
- preformatted fences (with language + key=val attrs), math blocks
  (with `%env%`), single- and multi-line comments, all four
  page placeholders (%title / %nohtml / %template / %date)
- inline: bold/italic/strike/super/sub delimiters, inline code, inline
  math, all six keywords (word-bounded), wikilinks (with description
  separator), transclusions (with attr separator), raw URLs across
  http(s)/ftp/mailto/file schemes — trailing sentence punctuation
  stripped from URL spans

Spans are byte-accurate per SPEC §6.5 (0-indexed line + byte column +
absolute byte offset). The lexer is permissive: every delimiter is
emitted, the parser will pair them and fall back to literal text on
mismatches.

Tests (41 cases) cover one example per construct in §9, plus
word-boundary keyword detection, list-vs-bold disambiguation,
indent-vs-list disambiguation, and a span-correctness check.

README now carries a per-phase status table; SPEC top-line status
advanced to Phase 3 complete.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/nuwiki-core/src/syntax/vimwiki/mod.rs

@@ -0,0 +1,8 @@
+//! Vimwiki syntax plugin.
+//!
+//! Phase 3 lands the lexer; the parser arrives in Phase 4 and the full
+//! `SyntaxPlugin` impl follows.
+
+pub mod lexer;
+
+pub use lexer::{VimwikiLexer, VimwikiToken, VimwikiTokenKind};