[Arfath Ahmed Syed (ArfiArfath21)]

Fixes #583

The TypeScript jinja2 key resolves to NunjucksRenderer, which diverged from the Jinja2 reference on two REQUIRED subset behaviours:

• {% if docs %} took the true branch for [] and {}, because nunjucks compiles conditions to JavaScript truthiness (spec/jinja-subset.md §2.2).
• trim / join on an omitted input and upper / lower / trim on a number threw TypeError, where Jinja2 renders "" or the stringified value (§2.1, §2.5).

What changed, all in runtime/typescript/packages/core/src/renderers/nunjucks.ts:

• While renderSafely compiles and runs a template, if, inline if, not, and and or are emitted around a runtime.truthy check that follows Jinja2 (empty list, map and string are falsy). and / or still return an operand, as in Jinja2. The compiler methods and runtime.truthy are put back in a finally, the same way the existing memberLookup / callWrap hooks already are, so other nunjucks users in the process are untouched.
• upper, lower, trim and join are overridden through env.addFilter to coerce first (undefined / null become "", numbers and booleans stringify, join accepts strings and maps the way Jinja2 does).

The sandbox hooks, the renderer key and the nonce handling are unchanged.

Tests (tests/renderers.test.ts): 23 new cases for truthiness (every falsy and truthy shape, elif, not / and / or, inline if, the guard-a-loop scenario), filter coercion, and restoring nunjucks afterwards, including after a template error. Their expected values are what Python's jinja2 renders. 9 of them fail on main and all pass here.

Verification:

• vitest in packages/core: 1257 passed. tsc --noEmit and npm run format:check are clean.
• Differential fuzz against Python jinja2: 6,000 random subset templates, then 10,000 more with empty-collection conditions, all identical after the change (before it, 167 of the 6,000 in the narrowest run differed). The Python and TS jinja_subset engines already matched jinja2 on all of them.
• I could not run the Java, C#, Swift or Go renderers here.

Happy to follow up with shared render vectors for empty-collection truthiness and undefined-through-filter, so the other runtimes are held to the same behaviour, if you want them.