fix(lexer): bare ${X:-${Y}} — extend VarRef to the balanced closing brace

[tobert]

Closes #173.

The bug

Bare ${X:-${Y}} was a parse error: the VarRef regex \$\{[^}]+\} stops at the first }, so it lexed as VarRef("${X:-${Y}") + RBrace. The quoted form always worked (the in-string interpolation parser hand-rolls brace matching), as did ${X:-$Y}. Found during the #95 census; pre-existing, unchanged by #172.

The fix

logos regexes can't count nesting, but a callback can extend a match: the pattern is now just \$\{ and lex_varref walks lex.remainder() counting brace depth, extending the token to the balanced } via lex.bump(). The parser needed zero changes — find_default_separator was already ${}-nesting-aware and default words already route through parse_interpolated_string, which is exactly why the quoted form worked all along.

Contracts preserved / clarified:

• ${#X} / ${#u[tags]} still lex as VarLength — its full regex out-matches the two-character opener in logos rule selection.
• Depth counting stays quote-blind, matching both the old regex and the scanner's ${...} region tracking (they use the identical algorithm on identical bytes, so they can't disagree on where a region ends).
• ${} stays an error (old regex required one inner char); unbalanced ${X:-${Y} is now a proper unterminated variable reference instead of a generic unexpected-character.
• ${a}b} closes at the first balanced } — pinned by a test.

$ kaish -c 'echo ${X:-${Y:-fallback}}'   # fallback
$ kaish -c 'Y=mid; echo ${X:-${Y:-fallback}}'   # mid
$ kaish -c 'X=top; echo ${X:-${Y:-fallback}}'   # top

Testing & review

• Full gates: cargo test --all (4,610 passed), clippy --all-targets 0 warnings, insta clean, no-default-features check.
• New tests: token-shape (single/double nesting, VarLength tie), error cases (unterminated, extra open brace, empty), early-close contract, and kernel-routed evaluation tests; nested example added to LANGUAGE.md's :- section (verified in the REPL).
• kaibo deepseek consult on the worktree: change judged sound on all five review axes (rule selection, bump math, scanner consistency, marker interaction, coverage); its one suggested pin (${a}b}) is included. CHANGELOG bullet under Unreleased/Fixed.

🤖 Generated with Claude Code