feat: add 7 builtins (cat, has, hd, tl, rev, srt, slc) + ternary guard-else

[danieljohnmorris]

Summary

• 7 new builtins: cat, has, hd, tl, rev, srt, slc (opcodes 49-55)
• Ternary guard-else: cond{then}{else} — produces a value without early return
• Each builtin has verifier type checks, interpreter implementation, VM opcode, and tests
• SPEC.md updated with all new builtins and ternary syntax

Builtins

Builtin	Signature	Description
cat xs sep	L t, t → t	Join list of text with separator
has xs v	list_or_text, any → b	Membership test
hd xs	list_or_text → any	Head (first element/char)
tl xs	list_or_text → list_or_text	Tail (all but first)
rev xs	list_or_text → list_or_text	Reverse
srt xs	list_or_text → list_or_text	Sort (numbers/text)
slc xs a b	list_or_text, n, n → list_or_text	Slice from a to b

Ternary

f x:n>t;=x 1{"yes"}{"no"}    -- returns "yes" or "no"
f x:n>n;=x 0{10}{20};+x 1    -- ternary value discarded, returns x+1

Test plan

• 917 tests pass (805 unit + 112 integration)
• Verifier, interpreter, and VM tests for each builtin
• Ternary tests: true/false branches, no-early-return, negated, guard unchanged
• Parser test for ternary AST structure
• No new clippy warnings

Supersedes PRs #49-#55 (which had opcode collisions from parallel development).