Fix variable ranges, for limit, if/for scoping, append coercion & string delimiters

[benfaerber]

Several fixes that bring liquid_ml in line with the Ruby/Shopify Liquid
reference implementation. Each is covered by new regression tests in
test/test_bug_fixes.ml; the full suite (incl. the real Shopify theme
fixtures) passes — 294 tests.

What's fixed

Variable ranges (1..n) / (a..b)

Previously only integer literals were accepted; a variable bound silently
produced an empty loop. LexRange now carries two lex_values, and a new
deferred Range of value * value is resolved at interpret time in
Values.unwrap. Descending ranges yield an empty list (matches Ruby).

{%- assign n = 4 -%}{% for i in (1..n) %}{{ i }}{% endfor %}   => 1234

for had an implicit limit of 50

Ruby's for has no default limit; only an explicit limit: caps it. The
default is now effectively unbounded.

if/for no longer discard in-body assignments

This matched neither Ruby nor Shopify: a variable assigned inside an
if/for body was deleted when the block ended. Ruby writes assign to the
root scope, so those values persist.

{% if true %}{% assign x = 5 %}{% endif %}{{ x }}                  => 5
{% for i in (1..3) %}{% assign last = i %}{% endfor %}{{ last }}    => 3

The loop variable and forloop are still scoped to the loop (restored to
their pre-loop state afterwards), and break/continue no longer leak past
the loop. Verified with nested loops and shadowed loop variables
({%- assign i='before' -%}{% for i in (1..3) %}{% endfor %}{{ i }} => before).

append / prepend coerce non-string arguments

Ruby coerces operands to strings. These now stringify the same way {{ }}
output does.

{{ 'n=' | append: 5 }}    => n=5

Lexer: delimiters inside string literals

The block-token scanner is now quote-aware: {{, }}, {%, %} inside a
string literal in a tag are no longer mistaken for block delimiters. Raw text
is unaffected (quotes there are not treated specially), so well-formed
templates are unchanged — this only makes the lexer accept a superset.

{% assign o = '{% raw %}' %}{{ o }}   => {% raw %}

Compatibility note

liquid_ml renders nil/undefined as the literal "nil" (Ruby renders "").
That is pre-existing and unchanged here, but append/prepend now surface it
({{ missing | append: 'y' }} => "nily"). I kept append consistent with
{{ }} interpolation, so fixing the global nil→"" rendering later would
make append Ruby-correct automatically. Happy to do that in a follow-up.

Files

• liquid_syntax/tokens.ml, liquid_syntax/syntax.ml, liquid_syntax/values.ml — Range value + resolution, default for limit
• liquid_parser/lexer.ml — range-bound atoms, quote-aware block scanner
• liquid_std/liquid_string.ml — append/prepend coercion
• liquid_interpreter/interpreter.ml — if/for scoping
• test/test_bug_fixes.ml (+ test/test_runner.ml) — 18 regression tests

[codecov]

Codecov Report

❌ Patch coverage is 87.12121% with 17 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
liquid_parser/lexer.ml	82.97%	8 Missing ⚠️
liquid_syntax/values.ml	55.55%	4 Missing ⚠️
liquid_std/liquid_string.ml	66.66%	2 Missing ⚠️
liquid_syntax/syntax.ml	66.66%	2 Missing ⚠️
liquid_interpreter/interpreter.ml	92.30%	1 Missing ⚠️

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